SKELETON. The word " skeleton," meaning in Greek a mummy, is popularly taken to denote that assemblage of bones and cartilages which forms the internal support of the body of man and of the animals more or less nearly resembling him. A slight acquaintance with the structure of these animals, however, seems to make it evident that a wider signification must be given to the term, since parts which in man and many of his animal allies are bony or cartilaginous may be only membranous in other such animals; and, con-versely, parts sometimes quite external, which are merely membranous in man and many animals, may in others assume the structure of horn or bone or may contain bones or cartilages. The word skeleton may indeed be taken to denote both a more or less firm and com-plete external protection to a living body, and also a more or less firm and complete internal support to such body.

In this very wide sense even many vegetal structures may be said to possess a skeleton. For all plants which can sus-tain themselves in an upgrowth from the ground obviously both require and possess solid structures—various groups and varieties of woody fibres—to support such an upgrowth. Organs also, such as leaves, which need to be maintained in the form of a thin flat expanse, require and possess bundles of fibres (vulgarly called veins) which are even popularly said to constitute the skeleton of the leaf. Many plants form such skeletal structures largely of silex, as do the grasses and the horsetails (Equisetwn), and others invest themselves to a greater or less degree with carbonate of lime, as do some Algx, such as Corallina and Melobesia, while the Desmidix clothe themselves with a horny coat. Ordinarily, however, the word skeleton is only used to denote certain animal structures, and mainly such structures as form the skeleton of man and of creatures so nearly allied to him as to constitute, together with him, that primary division of animals known as backboned animals or Vertebrata.

It is to a concise description of the skeleton as it exists in Vertebrates generally that this article is devoted. For the details of the human skeleton the reader is referred to the article ANATOMY. In order, however, that its condition in Vertebrate animals may be better understood, it will be well briefly to point out some of the more important varieties of condition presented by the protecting or sup-porting parts of the body of the lower, or Invertebrate, animals.

THE SKELETON OF INVERTEBRATA.

A great and fundamental distinction exists, however, between those lowly organisms known as Protozoa or Hypozoa—which are generally reckoned as animals—on the one hand and all the higher forms, both Vertebrate and Invertebrate, on the other. It is a dis-tinction which renders it difficult to regard any skeletal structures of the Hypozoa as answering to, in the sense of being the homologues o f, any of the skeletal structures of higher animals. This great funda-mental distinction consists in the fact that the bodies of all the higher animals are made up of distinct "tissues," which are derived from three different layers of cells, of which the embryos of all of them are for a time composed, whereas the bodies of the Hypozoa either consist of but a single cell or else of a smaller or larger number of cells more or less loosely aggregated and not forming any distinct tissue. It follows of course that their reproduction does not take place by means of embryos formed of cellular layers.

Nevertheless the Hypozoa or Protozoa may exhibit very distinct protective structures. Thus the outermost layer of the substance of an Amoeba, called its ectosarc, is of a firmer consistency than its interior, and it may in allied forms take on a chitinous character or become quite hard through the deposition within it of calcareous salts (as in the sometimes singularly complex shells of the Fora-minifera) or form symmetrical eases of silica.

In the Badiolaria, the skeleton of the Protozoa attains its maxi-mum of beauty and complexity. It consists of spicules which are generally siliceous, but may consist of a peculiar firm organic substance termed " acanthin." The spicules arrange themselves in an extraordinarily symmetrical manner, generally radiating from the central portion of the organism and being connected with one or more series of encircling spicules which may constitute a series of concentric spheres.

Among the Infusoria we also find examples of a hardening of the external cuticle, as in Tintinus lagcnula and in some other forms.

When we pass to that vast group of animals—the Metazoa— which includes all but the Protozoa (and all those therefore the bodies of which are formed of tissues derived from the three primitive layers), a distinction again requires to be drawn between the Sponges (Porifera), which constitute its lowest group, and all higher forms. The three primitive or germinal layers of the Metazoa are termed respectively—(1) the epiblast, (2) the mesoblast, and (3) the hypoblast. Of these three layers the epiblast and the hypoblast are to be regarded as primary. The epiblast is essentially the primitive integument, and its cells give rise to the epidermis and cuticle and to the organs of sense. The hypoblast is essentially the digestive layer, and gives rise to the epithelium lining the aliment-ary canal. The mesoblast seems to originate from one or both of the two preceding layers, and gives rise to the general substance of the body—including that part of the skin which is beneath the epi-dermis, the muscles, and the blood-vessels. It may divide into two layers, whereof the more external is distinguished as "somatic," while the more internal is called '' splanchnic." Such is the general condition of the three germinal layers in the Metazoa. In the Sponges, however, it seems probable that the germinal layers have a different nature—the epiblast and mesoblast being respectively the digestive and sensory layers.

The skeletal structures of the Sponges have the form of spicules, which may vary greatly in different genera as to their form, while they may be siliceous, calcareous, or horny. Sometimes they con-stitute structures of singular beauty. They appear to be formed in or on the cells of the mesoblast, and it does not seem that any skeletal structures arise in the epiblast or hypoblast of the Porifera. Should such, however, be hereafter found, then it must be borne in mind that their homologies with analogous skeletal structures of other organisms must depend on the final decision of the question of the exact relations which may exist between such germinal layers in Sponges and the epiblast and hypoblast of higher Metazoa.

In the great group of the Ocelentera, the skeleton may be either epiblastic or mesoblastic in nature. Thus in the Hydrozoa—where it mostly has the form of a horny investment, but may be (as in the Millepores) calcareous—it is epiblastic. In the Actinozoa— which includes the true coral animals—it is generally mesoblastic, although it is formed from the epiblast in the Gorgonise, Isidinee, and Pennatulidse.

In Isis the skeleton curiously consists of a series of segments which are alternately horny and calcareous.

In the Echinodermata we generally have, notably in the Sea-Urchin (Echinus), a wonderfully complex skeleton, which is so near the outer surface that at the first glance it seems necessarily a most external form of skeleton. Nevertheless the plates which compose it are mesoblastic in nature and are independent of the epidermis.

The two valves forming the shell of the Lamp-shells (Branchiono-poda), and the very different two valves which constitute the shells of creatures of the Oyster class (Lamellibranchiata), as well as the single shells of the Snail and Whelk class (Gasteropoda), are all epiblastic in nature, and are calcifications of the outer part of the epidermis. The same is the origin of the apparently internal shell of the Slug, which is at first external in the embryo and subsequently becomes enclosed.

Similar is the nature of both the internal and external shells of the Squids, Cuttle-fishes, and Nautili, i.e., of the class Cephalo-poda. In the last-named class, as in some Gasteropods, there is a cartilaginous structure inside the head, which structure supports and partly protects the brain. It is unlike any skeletal part yet mentioned save in its mode of origin, which, like the skeleton of some of the Actinozoa, is mesoblastic.

Lastly may be mentioned the hard protecting external coat of insects and animals of the Crab and Lobster class—in short, the external skeleton of that primary division of animals which is called A.rthropoda. This is again epiblastic, and a hardening of a cuticle on the outer surface of the epidermis—a hardening effected gene-rally by chitinization (the deposition in it of a substance termed "chitin"), or, as in many Crustacea and some Myriapoda, by calcification.

GENERAL SKELETAL CONDITIONS.

Having thus briefly glanced at the leading skeletal structures of a number of groups of lower organisms, we may make the following generalization, which will be of use to us in helping us to understand how the skeletal parts of backboned animals stand related to the skeletal parts of animals lower in the scale :—

(1) Skeletal structures may conceivably arise in parts which are epiblastic, or mesoblastic, or hypoblastic.

(2) Skeletal structures belonging to any one of those three categories may be further divisible into two subordinate categories according as they belong to a superficial or a deep part of the layer to which they appertain.

(3) Skeletal structures may be siliceous, chitinous, calcareous, cartilaginous, or horny.

(4) In certain animals the mesoblast subdivides into two layers, one somatic and the other splanchnic. Obviously, then, there may be skeletal parts corresponding to either of these last-named layers, and conceivably to a deeper or more superficial portion of either of them.

THE SKELETON OF VERTEBRATA.

The skeleton of the Vertebrata—that is, of the five classes of animals named Pisces, Amphibia, Reptilia, Aves, and Mammalia—may in the first place be most conveniently considered as consisting of two parts—a dermal skeleton, or exoskeleton, and an internal framework, or endoskeleton. The latter, which is generally much the more considerable, is mesoblastic, and the muscles are external to it.

EXTERNAL SKELETON OF VERTEBRATA.

This division of the skeleton is itself again made up of two parts. The more external of these is the epidermis and is of epiblastic origin, and dense epidermal structures may arise towards its inner or its outer surface. The more internal constituent of the exoskeleton is the dermis and dense structures formed in it, and these are from the outer portion of the mesoblast.

Epidermal hard structures formed towards either sur-face of the epidermis may become intimately united with subjacent dermal hard structures, and then again, as we shall see, with parts of the true endoskeleton.

Any hard structures formed in the walls of the alimentary canal—the lining of which is continuous at either end with the external skin—are to be reckoned as fundamentally exoskeletal. In the process of development the epiblast becomes inflected more or less into either extremity of the alimentary tube, but the intermediate portion, together of course with any hard structures de-veloped in it, is of hypoblastic origin.

In the great majority of Vertebrate animals the two layers of the skin, the epidermis and the dermis, are, as in man, soft, though locally provided with certain denser appendages, such as epidermal and dermal scales, hairs, nails, scutes, and teeth.

The soft, general exoskeleton or skin invests the body of Man pretty closely, though slightly projecting folds of it extend between the roots of the fingers and toes. In some abnormal cases these folds extend so far and bind the digits together so much that the thus malformed person is said to be " web-fingered " or " web-toed." Such a condition is found normally in many animals, as notably in Ducks and Geese, and such parts form a large portion of the " wing " of the Bat.

Other extensions of the skin of the body are note-worthy. Thus in the " Flying " Squirrels and Opossums, and the curious Bodent named Anomalurus, the skin of the sides, between the arms and the legs, is much expanded, serving for a parachute. There may be a skin parachute supported by long free movable ribs, such as we shall see exist in the little Lizards called " Flying Dragons." There may be a very remarkable extensive skin round the neck, as in the Frilled Lizard, and folds of skin may hang freely, as in the " dewlap " of Cattle, or may be formed here and there as in the Rhinoceros, the skin of which animal is so thick as to necessitate the existence of such folds to allow free movements to the body and limbs. Long filamentary processes may be formed along the back, as in the Iguana and various other Lizards.

In the Seals a fold of skin connects together the hind legs and the tail, and also in our common Bats, which have in addition their very elongated webbed fingers connected with the sides of the body and legs by another great fold of skin which, with those between the fingers, forms the entire bat's " wing."

The integument may be very distensible, as in those Fishes (e.g., Diodon) which distend themselves with air and then float belly upwards.

The epidermis of many Vertebrates, and of Man, is shed in minute fragments, constantly removed by friction and ablution, and constantly replaced; only under abnormal conditions and after certain diseases does it come away in large and continuous patches. In some other Vertebrates, as notably in Snakes, the entire epidermal investment of the body, even that of the eyes, is cast off entire as one whole.

The epidermis never has its superficial layer connected with bone, but it often becomes thickened and horny, as we see in the sole of the foot, or the labourer's hand, and in those abnormal thickenings called " corns." Certain local thickenings which are not abnormal may exist in animals; such are the callosities on the inner side of the legs of the Horse, on the breast of the Camel, and on the nates of the lower Old-World Apes.

Of the appendages of the epidermis the most simple are scales, such as we find on the legs of Birds and the bodies of Serpents and Beptiles generally.

A scale—a true scale, such as those of Snakes and Lizards— consists of papillse of the dermis invested by the epidermis, the whole being covered by a cornification of the external part of the epidermis. Scales may be very diverse in shape, prominence, and relative size, and may form very large plates. The so-called scales of Fishes are of deeper origin and are a form of scutes.

A hair differs from a scale in that, instead of being an epidermic investment of a dermal projection outwards, it originates by an epidermal projection inwards into the subjacent dermis. A small papilla of the dermis, however, soon projects upwards, in turn, into the descending epidermal process, and then cornification sets in (at first in the immediate vicinity of the dermal papilla) in the cells around the axis of the epidermal descending projection, and this hardened portion soon projects beyond the surface of the body, while the part of the epidermis about its deepest part becomes modified into its so-called "root."

A nail or claw arises as a cornification of the epidermis (but not of its deepest layer) lying upon numerous very vascular ridges (or transversely elongated papillse) of the dermis, forming the primitive bed of the nail, and enclosed in a deep fold of the integu-ment. One end of the structure becomes free and projecting superficially, while the opposite region grows by epidermal additions from beneath and at its attached extremity.

A feather is more nearly related to a scale than it is to a hair. It consists at first of an upwardly-projecting dermal papilla invested with epidermis, and it is only at a later stage that its base sinks into a sack or "feather follicle." The outermost layer of epidermis becomes converted into a horny sheath, which is thrown off when the feather is completed. The quill is formed by cornification of the deepest and more superficial layer of epidermis investing the base of the dermal and vascular papilla, and is open at both ends. The vascular papilla it encloses shrinks up when the feather is fully formed. The vane of the feather is formed from the more apical portion of the papilla, and its central part, or shaft, is con-tinuous with the quill, while ridge-like thickenings of epidermis diverging from either side of this central part constitute the barbs of the vane, from each of which yet smaller processes or barbules proceed.

A scute is a hardening of the outermost portion of the dermis, with an investment from the deepest layer of the epidermis. Such are the so-called scales of ordinary Fishes, which may be represented by the bony plates and processes called placoid scales—so common in the groups of Sharks and Rays. In these latter structures dermal papilla? appear and calcify, forming a dense structure with-out corpuscles, called dentine, beneath which may be a corpusculated structure of true bone. The calcifying papilla? receive an invest-ment of still denser calcareous tissue, called enamel, from the deepest layer of the epidermis. These placoid structures often come to project outwards on the surface of the body as long spines or as shorter tooth-like processes, or they may protect the surface of the body as flat plates. Often the dentine more or less entirely atrophies, so that the structure comes to be formed almost entirely of true bone or of that peculiar calcified tissue of wdiich the scales of ordinary Fishes (such, e.g., as the Perch and Carp) are composed.

A tooth is a structure closely related to a scute. It differs from the latter just as a hair differs from a scale—namely, by owing its origin to an ingrowth of the epidermis instead of merely to a primitive outgrowth of the dermis.

The so-called teeth of the Lamprey are not true teeth, but are merely horny epidermal structures essentially similar to scales.

In the origin of a true tooth a process of the epiblastic layer of the mouth—the buccal epithelium—grows into the subjacent dermis, and, assuming a cup-like form (with the concavity of the cup turned away from the epithelial surface of the mouth), a dermal papilla rises into the cup. The apex of this papilla then superficially calcifies into dentine, and becomes invested with a layer of enamel formed from the immediately adjacent surface of the epidermic cup or " enamel organ." An investment of connective tissue called the dental capsule becomes formed round the whole. The dentine then increases, a remnant of the papilla remaining as the "pulp." The young tooth gradually approaches the buccal surface, and the base of the papilla becomes formed into the root or fang of the tooth. The enamel organ does not descend so far, but only invests the crown of the tooth. The inner layer of the capsule, however, investing the fang gives rise to a third dental tissue known as the cement. A bud may or may not be given off from the developing tooth to serve as its future successor.

Thus teeth are normally both epiblastic and mesoblastic struc-tures, but in certain Fishes they line parts of the throat (the branchial arches), the superficial membrane of which is derived from the hypoblast, and such may of course be considered as hypoblastic skeletal elements, and, thus considered, must be reckoned as constituting a separate category of teeth.

Such being the various kinds of dense structures which enter into the composition of the Vertebrate exoskeleton, each kind may be developed to a greater or less extent in different groups of Vertebrate animals.

Exemplifications of Epidermal Skeletal Parts.

Scales entirely clothe the bodies of most Lizards and Snakes and the legs of Birds. In Tortoises and Turtles they take the form of large plates, which in one species are known as tortoise-shell. The shape and size of scales are made great use of as distinctive characters for classification. See REPTILES. The scales of a Serpent are held together by their epidermic investment in such a way that it and they are cast off as one whole each time the animal effects that process known as changing its skin. In the Rattle-snakes curiously modified thickenings of epidermis surrounding the end of the tail are not cast off but continue partially adherent; as growth proceeds and successive castings of the skin take place, these ring-like thickenings become numerous, and so knock one against the other, when the end of the tail is vibrated, as to pro-duce a singular sound—the so-called rattling of the system of rings or "rattle."

Hairs form the characteristic clothing of the class Mammalia, though certain Mammals, such as Whales and Porpoises in their adult condition, are naked. Man is quite exceptional in having the ventral surface of the body more hairy than its dorsum. Long hair on the head, and whiskers and beard, are variable human characters, also possessed by some Apes ; and many animals—as the Lion, the Horse, the Aardvark, &c.—have long hair in one or other region of the body. Some hairs may be especially thickened and serve as feelers, as in the "vibrissae" or "whiskers" of the Cat tribe. But the maximum of development is shown in such creatures as the Hedgehog and the Porcupine, where hairs become dense and solid spines.

Nails do not exist in the class of Fishes and rarely in that of Batrachians. They first make their appearance in the most simple form—that is, in the form of slight thickenings of the epidermis— at the ends of the digits in certain Toads and of one kind of Eft. A nail is at its maximum of development when it quite surrounds and encloses the last or end bone of the digit which bears it. Such nails exist in Horses, Oxen, &c, and are called hoofs. A nail when produced into a sharp point is called a claw,—as in the familiar case of the Cat, and also in Birds. Nails may, how-ever, be much reduced in size and not nearly extend to the end of the digits which support them, as in the Sea Bears. They may be altogether wanting, even in Mammals, as in the Porpoise, or attain a prodigious relative size, so that the body can be suspended by them in progression, as in the Sloth.

Nail-like structures may be developed from the side of the hand, as in certain Birds (e.g., Palamedea), which are said to be "spur-winged," and in a Mammal (Omithorhynchus) a hollow horny spin-grows upon each ankle.

In the Rhinoceros we meet with a horn, or two horns, which grow up from the dorsum of the muzzle like a great blunt nail, long dermal papilla? extending into it and answering to the dermal ridges beneath a true nail. In Owen's Chameleon no less than three long horns are developed—one from the nose and a sym-metrical pair from the front of the head.

Other horns which do possess bony cores are developed from the head in pairs on the so-called hollow-horned Ruminants, i.e., the Oxen, Antelopes, Goats, and Sheep ; and only in one anomalous form, the Prongbok (Antilocapra), are these horny structures shed at intervals ; in the rest they persist throughout life. Normally there is never more than one pair amidst existing Ruminants, with the exception of the Four-horned Antelope, which has two pairs. Such horns may be straight or curved or spirally twisted, but they are never branched, with the single exception of the Prongbok.

Sharp-edged, overlapping, horny plates (each of which is com-parable with a nail) may be developed beneath the proximal part of the tail, as in the curious Rodent Anomalurus. Such plates may clothe the entire body, head, limbs, and tail, as in the scaly Manis or Pangolin.

The epidermis and epithelium which respectively line the out-side and inside of the jaws may both be converted into horn, forming a small beak which may be composed of a number of close-set processes and may be temporary, as in the Tadpole, or permanent, as in the Siren. Larger and denser structures of a similar kind form the beak of Birds and of the Turtle and of that most exceptional Mammal, the Omithorhynchus.

The epithelium within the mouth may be locally cornified, forming horny teeth which have, as before mentioned, rather the nature of scales—as in the suctorial mouth of the Lamprey.

In certain Beasts, as the Cow and the Sheep, the front edentulous part of the upper jaw is invested by a horny epithelial pad against which the teeth of the front of the lower jaw bite. A much more developed structure is met with in the Dugong. The front of both jaws is furnished with a dense horny plate formed like the horn of the Rhinoceros, though of course widely different in shape. But the maximum development of this kind of structure is found in the Whalebone Whales. The upper jaw in these is furnished with very numerous horny plates, termed baleen, which hang down from the palate along each side of the mouth. They thus form two longitudinal series, each plate of which is placed transversely to the long axis of the body, and all are very close together. The outer edge of each plate is entire, but its inner edge gives forth numerous hair-like processes. These are some of the constituent fibres of the horny plates which thus, as it were, fray out and line the sides of the buccal cavity with a network of countless fibres formed by the inner edges of the two series of plates. This network acts as a sort of sieve, allowing water to escape between the plates but retaining in the mouth the small creatures on which the whale feeds.

Cornifications of the tongue may exist. Thus in some Birds, as in Woodpeckers, the structure of its apical portion becomes so dense that it serves as a dart or spear. Its surface may be more or less cornified in Beasts. Thus it may be furnished all round with backwardly-pointingspines, as in the Lesser Anteater (Tamandua). There may be a large horny papilla on each side of it, as in the Manatee or Omithorhynchus, or there may be horny plates on the tongue, as in the Java Porcupine.

Homy structures also exist which cannot be considered as either epiblastic or mesoblastic, but must be hypoblastic in origin. Such are the horny linings of the stomachs or gizzards of Birds, and the similar lining of the stomach of the Great Anteater, Myrme-cophaga jubata.

Feathers are the universal and peculiar cutaneous appendages of Birds, and generally differ much in size in different parts of the body, long and strong feathers constituting the most conspicuous part of the wings and so-called "tails" of Birds. Feathers are implanted on the body neither in an irregular nor in a uniform manner, but are aggregated together in different modes in different groups of Birds—each definite patch of implanted feathers being called a feather tract. The arrangement of these tracts in a bird is called its " pterylosis," and serves amongst other characters to distinguish different groups of Birds one from another.

Exemplifications of Dermal Skeletal Parts.

Scutes.—True dermal ossifications are met with in some kinds of Mammals. Thus the Armadillos possess a very complete external dermal skeleton formed of small many-sided bony scutes, the margins of which are adjusted together, and which are differently aggregated—into transverse bands or into larger inflexible masses —in different species. In the extinct Glyptodon, the body was invested, from the neck to the root of the tail, with one such solid case.

In the Armadillos a horny epidermal skeleton is so adjusted to the bony case that the former is divisible into small scales corre-sponding with the several scutes. Amongst Reptiles, we find in the Tortoises and Turtles (e.g., Emys, Testudo) a solid exoskeleton, the dorsal part of which is called the '' carapace," wdiile the ventral portion is named the "plastron." The former consists of a median series of scutes, to each side of which is annexed a series of lateral scutes which are more elongated transversely to the long axis of the animal's body, and these three series are intimately united with subjacent portions of the internal skeleton. The carapace is completed by a series of smaller scutes, which surround it and are therefore called " marginal'' scutes. The plastron consists of eight pairs of scutes and one azygous scute. In the Box-Tortoises the ends of this plastron are movable, and (the head and limbs of the animal being drawn in within the shell) can be applied to the ends of the carapace, so that all the soft parts can be completely enclosed within the dense exoskeleton. As in the Armadillos, the bony scutes are covered by epidermal scales, some of which have been already referred to as constituting "tortoise shell." Unlike the Armadillos, however, the segments of the epidermal and dermal skeletons do not correspond. The dorsal scales are much larger and less numerous than are the scutes, but, while the scutes of the plastron are but nine in number, it has twelve horny plates or large scales.

Amongst the Amphibia certain Frogs (e.g., Ephippifer and Ceratophrys) develop dorsal osseous scutes, and these, as in the Tortoises, are more or less united with parts of the subjacent internal skeleton.

A solid skeleton of juxtaposed osseous scutes may exist in Fishes, as in the Bony Pike Lepidosteus, where the scutes are enamelled and united by a peg-and-socket articulation. Polypterus also has an investment of bony scutes, and in the extinct fish Pterichthys they were developed into large plates on both the dorsal and ventral surfaces of the body. The Sharks and Rays may have their scutes thickly distributed over the surface of the body, but quite small. A skin so furnished is called "shagreen." They may also be larger and fewer, and placed far apart, with elegant patterns on their exposed surfaces; or they may take the form of strong defensive spines. In the Sturgeon the scutes are arranged in rows along the body, separated from each other by softer portions of integument.

In the ordinary bony Fishes, or Teleostei, the scutes (commonly but erroneously called " scales ") are differently calcified from the scutes of Sharks, and may have their free projecting margin smooth, when they are described as cycloid, or in toothed-like processes, when they are termed ctenoid; or they may be inter-mediate between these two types of form. The Teleostean scutes are generally separate, but they may coalesce to form a connected solid investment, as in Ostracion and the Seahorses (Lophooranchii), or develop strong projecting spines, as in Diodon.

Fishes have two other very important exoskeletal structures, which may be bony or cartilaginous. One set of these structures consists of filamentary processes, which may be either horny or calcareous, and which support the skin of the fins, whether those of the back, belly, and tail, or those of the limbs ; such structures are termed " fin-rays." The other set consists of bony or cartilagin-ous hard parts, which serve to support the fin-rays, which therefore lie more deeply, or at least are less projecting, and are commonly termed "interspinous bones or cartilages," but which may be con-veniently distinguished as radials; they are very important elements of the fins of Elasmobranchs.

Certain Siluroid fishes exhibit in the adjustment of portions of their dermal exoskeleton an altogether peculiar mode of articula-tion, called a shackle joint. This is in the form of a dermal scute articulated with a superposed spine. The scute has an osseous ring on its dorsal surface, and through this passes another osseous ring which forms part of the base of the superimposed spine.

In connexion with dermal scutes and spines may be mentioned those familiar yet exceptional structures, the bony horns of Ungu-lates. In the Oxen, Goats, and their allies horns exist on the head as bony cores, persisting throughout life, and supporting those " hollow horns " before noticed amongst the epidermal or epiblastic parts of the exoskeleton. As is the case with the scutes of Chelo-nians, these bony parts are intimately united with subjacent parts of the true endoskeleton. In the Giraffe there are three such bony prominences, which arise as distinct ossifications, and only later anchylose with the skull. These are the Giraffe's pair of short horns, together with the median prominence in front of them. In the Deer we find bony antlers, which are shod annually and are destitute of any horny covering. Antlers may exist in both sexes, as in the Reindeer, but generally they are present in the males only. They arise as soft highly vascular prominences, and when fully grown become hardened by calcareous deposit. In some months the investing skin dries up and is got rid of; and the horn itself falls off after the breeding season, leaving a stump whence a new antler shoots forth again in the following year. Antlers, as a rule, are branched—more so as the individual becomes older, till maturity is attained. Some Deer have enormous antlers, weighing as much as 70 lb, and formed at the rate of 1 lt> a day.
Teeth.—The differences in structure, number, form, and develop-ment of the dental organs are so great that they cannot here be treated of. See vol. vii. pp. 232 sq.; also vol. xv. pp. 349 so/.

INTERNAL SKELETON OF VERTEBRATA.

The most essential part of the Vertebrate internal skeleton is the spinal column, the foundation of which is laid by a temporary or permanent structure called the notochord or chorda dorsalis. At the anterior end of the spinal column there is almost always a solid structure known as the cranium or skull, to which mandibular, hyoidean, and branchial arches may or may not be attached. The spinal column may be divisible into cervical, thoracic, lumbar, sacral, and caudal portions, and may have pro-cesses projecting from it upwards, downwards, or laterally, with arches of varying extent, as neural arches, chevron bones, and ribs, together with a median ventral portion— the sternum. The whole of these parts taken together constitute the axial skeleton. This may exist alone if the body is limbless, but otherwise additional hard struc-tures are found which together constitute the appendicular skeleton.





Vertebrate animals never have more than two pairs of limbs, and each pair is attached to the body by the help of certain skeleton elements termed a limb-girdle, diverg-ing from which are the hard parts which constitute the skeleton of either "appendage" or "limb." In addition to these we find in Fishes certain azygous structures—the unpaired fins,—the osseous or cartilaginous supports of which must be reckoned as a part of the appendicular skeleton. With the occasional (or possibly constant) exception of the notochord, the whole Vertebrate internal skeleton is a mesoblastic structure. In the great majority of the Vertebrata the skeleton is more or less bony, but it always in part consists of cartilaginous and fibrous structures.

The number and nature of the solid parts vary with age in the same species. When, in the earlier stages of existence, the process of ossification' has once begun, it goes on more or less rapidly till maturity is attained, and is continued, to a certain extent, throughout the whole of life.

The points at which bone formation begins and whence it radiates are termed " centres of ossification," and there may be one, two, or several of these in what is ultimately to become a single bone. Sometimes these " centres" have an important morphological significance, and in other instances they would seem to be determined by the size of the future structure. Bones are classed as " cartilage bones " or " membrane bones " according as they are formed either through the previous formation of a cartilage which subsequently ossifies or directly from membrane without the intervention of cartilage. These two classes can generally be easily distinguished, but there are instances in which it would seem that what is really the same corresponding bone differs as to its mode of origin in different animals. Moreover, a compound bone, formed of a membrane bone and a cartilage bone intimately united, may come to lose either its cartilaginous or its membranous elements, and thus further difficulties of interpretation may arise. There are also cases (as in the carapace of Chelonians) in which exoskeletal dermal bones coalesce with subjacent bones of the endoskeleton. Such bones may become deeper in position as development advances, and there is reason to think that not a few bones ordinarily reckoned as parts of the endoskeleton are of dermal origin, and first appeared in ancestral forms as placoid scutes or dermal spines.

As the development of the skeleton proceeds, ossification tends to fuse together more and more bones which at their first appearance were separate and distinct. This is notably the case in warm-blooded animals, and is most noteworthy in the warmest-blooded class—that of Birds.

Besides the coalescence of distinct bones, another fusion of bony structures occurs. This is due to the fact that the ends, or projecting portions, of what are essentially and ultimately one bone may for a time persist as distinct bony parts, termed " epiphyses." Thus, in the case of Man, the ends of the long bones of the limbs are at first separate from the main part (or shaft) of each long bone, and do not become continuous with the latter till the human frame has nearly attained maturity.

The hard parts of the internal skeleton, being those which as a framework support the body, form points of attachment for the muscles which move the body,—such hard parts being used as either levers or fulcra, as the case may be. The great majority of the bones are thus in-tended to move one upon another. The contiguous surfaces of bones form "joints," which may be immovable, mixed, or movable. The bones of the skull are united by immovable joints, called " sutures." Joints are said to be mixed when the motion allowed is exceedingly slight, as when two bones are allowed to be slightly separated from each other by the intervention of a softer substance which is attached to both. We have examples of movable joints in the human neck, the two uppermost bones of which are articulated on the principle of a pivot; in the elbow, which forms a hinge ; and in the shoulder, where the upper arm joins the shoulder-blade in a ball and socket joint.

If one convex articulating surface be globular, it is termed a head; if it be elongated, it is called a condyle. If either of these is borne upon a narrow portion of bone, this latter is called a neck ; if a pulley-like surface is formed by such a juxtaposition of two condyles as to leave a depression between them, such an articular surface is named a trochlea.

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The curious and exceptional arrangement termed a shackle joint has been already noticed under the head of "Scutes."

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AXIAL SKELETON.

The whole axial skeleton—including both the cranium and the spinal skeleton—apart from the notochord, is formed from the mesoblastic tissue bordering the medullary groove of the embryo. As the essential part of the axial skeleton is the spinal column, so the essential foundation of this column itself is what is known as the "notochord." This is an elongated cylindrical rod of soft tissue running along the antero-posterior axis of the body immediately subjacent to the central portion of the nervous system. Its mode of origin from the germ-layers of the embryo has yet to be finally determined. It is said by Balfour to be developed, in most if not all cases, as an axial differentia-tion of the hypoblast. The cells of the notochord form a tissue resembling cartilage, and it becomes surrounded by a more or less dense fibrous sheath. Such an organ is found to exist, temporarily or permanently, in certain lower creatures—Ascidians—which in most other respects widely differ from Vertebrate animals. Some few of these animals are furnished with a tail throughout the whole of life, v. die others are furnished with such an organ only in their larval or immature condition. It is alone in such permanent or temporary tail, and not in the body of As-cidians, that a structure of this kind is met with.

In every Vertebrate animal the notochord is the first part of the skeleton to appear, and it extends throughout the whole length of the body, as well as of the tail. In every such animal, except the Lancelet (Amphioxus), it becomes arrested anteriorly in the midst of that second-arily formed skeletal region which becomes the skull. In Amphioxus, however, in which no skull is ever formed, the notochord extends to quite the anterior end of the body. It is enclosed in a strong sheath, within which its substance is segmented so as to resemble a longitudinal series of coins or counters. The only other representatives of the internal skeleton in this animal are—(1) longitudinal ligaments (strengthening the sheath of the notochord above and below); (2) fibrous septa which pass out laterally from it between the muscles of the body, to the fibres of which they give attachment; (3) a longitudinal membranous sheath of the central part of the nervous system, forming an elongated antero-posteriorly directed cylinder above the notochord; (4) two vertical septa,— one dorsal, ascending medianly from such neural sheath, and one ventral, descending medianly from the sheath of the notochord in the region of the tail; (5) two jointed cartilaginous filaments which lie one on each side of the longitudinal slit which serves the lancelet for a mouth; and (6) certain cartilaginous filaments which strengthen the sides of the branchial cavity between the intervening vertical fissures of the walls of that cavity.

In all other Vertebrate animals the axial skeleton is divisible into that of the head, or the cranial skeleton, and that of the axial skeleton behind the head, or the spinal skeleton.

Spinal Skeleton.

In all Vertebrate animals except the Lancelet, the axial skeleton is complicated by a longitudinal series of additional hard parts—cartilaginous or osseous—which serve to protect the spinal cord, or marrow, above it, or the great blood-vessels beneath it, and which hard parts support, encroach upon, or replace the notochord itself. Nevertheless, the notochord persists throughout the whole of life in certain Fishes both of the lowest and highest types of piscine organization, but it does not persist in its entirety in any adult Vertebrate which is not a Fish.

In the Lamprey the notochord persists, but a longitudinal series of small, similarly shaped cartilages strengthen the sides of the more anterior part of the membranous dorsal canal which encloses the spinal marrow. In the Chimsera these are more developed, while numerous circular calcifications appear in the notochordal sheath. In the most anterior part of the trunk the cartilaginous elements unite to form a continuous investment of the notochord. Amongst the Ganoid Fishes, the notochord persists un-constricted and cylindrical in the Sturgeon and the Lepidosiren, but cartilaginous or bony parts appear about it and form a longitudinal series of arches above and below it for the protection respectively of the spinal marrow and sub-vertebral blood-vessels. In different kinds of Sharks further complications arise, and the notochord becomes encroached upon, in different modes, by chondri-fication and calcification, till it becomes segmented by the intervention of a series of thus formed hard parts called " bodies" or " centra," between which relics of the notochord still remain. By this process of segmentation there come to be formed what are called vertebra, the presence of which in the overwhelming majority of Fishes, as well as in all the higher classes of animals, has led to the whole group being called Vertebrata.

In the vertebrae of most Vertebrates we have a solid body or centrum, from the dorsum of which there arises on each of its two sides a neural plate, which then bends inwards to meet its fellow of the opposite side, thus form-ing an arch (the neural arch) for the protection of the spinal cord, or marrow, which passes through it. From the dorsal side of such neural arch a process called the neural spine very commonly ascends. From the sides of the centrum or neural arch, or of both, a single process, or two superimposed processes, may jut outwards, which are known as the transverse process or processes, to which the ribs are generally articulated when ribs are present. Inferiorly directed processes, single or double, may descend from beneath the centrum, or may be developed in the intervals between adjacent centra, and are generally related to the protection of large blood-vessels, though they may only serve for muscular attach-ment.

Adjacent vertebra are commonly connected together by special modifications of the neural arches or the centra, or of both. Mostly the opposed margins of the neural arches develop special processes for attachment called articular processes or zygapophyses, and there may be additional interarticulations. There may be as few as ten or as many as four hundred vertebrae.

Vertebrae may be divisible, as in the highest animals, into five categories:—(1) cervical, or those of the neck; (2) dorsal, or those of the back; (3) lumbar, or those of the loins; (4) sacral, or those with which the pelvic limbs are connected; and (5) caudal, or those which, are posterior to the sacral vertebra, or which support the tail when such an organ is present. There may be only two categories (dorsal and caudal), as in Fishes.

In most Fishes and some exceptional Reptiles the body or centrum of each vertebra is so imperfectly ossified as to remain biconcave or amphicoelous,—that is to say, it presents a deeply concave cup-like form both in front and behind. The space thus enclosed by the adjoining cups of each pair of successive vertebra is filled up by a soft, spheroidal remnant of the notochord, which thus serves as an intermediate connecting substance. The cups may become filled up by ossification, as in Man and Beasts, the flattened surfaces being connected by what are called inter-vertebral disks. Each such disk is made of fibrous lamella which surround a soft elastic central portion which is a last remnant of the notochord. Often the vertebra may have the centrum very convex at one end and very concave at the other, and so give rise to a ball-and-socket joint at each junction between the successive centra. Such vertebrae may be proccelous (i.e., have the cup in front and the ball behind), as in existing Crocodiles, or opisthoccelous (i.e., with the cup behind and the ball in front), as in the Bony Pike Fish (Lepidosteus), the Land Salamander, and the cervical vertebra} of Buminants ; sometimes a vertebra may be biconvex (i.e., have a ball at each end of its centrum), as in the first caudal vertebra of the Crocodile; or, very rarely, there may be two prominences, or the cups may exist side by side on one surface of a centrum, as in some cervical vertebrae of Chelonians. Instead of intervertebral disks, with spheroidal remnants of the notochord, adjacent vertebrae are often (as in Snakes) united by what are called synovial sacs, or membranous closed bags containing an albuminous fluid called " synovia " and commonly known as "joint-oil."

The various parts of a vertebra may be all united to form one single bone, as is generally the case in the higher animals, but such is by no means universally the case. In the Ichthyosaurus we find the neural arch permanently distinct from the centrum; and in the Carp the transverse processes are separate. The neural arch itself may be made up of two separate pieces on each side, as in some Elasmobranch Fishes, e.g., Raia and Spinax.
Sometimes the neural arch, instead of reposing upon its own centrum only, appears, as it were, shifted so as to be connected with two adjacent centra, as is the case, e.g., with the dorsal vertebrae of Tortoises.

Generally the nerves which pass outwards from the spinal marrow which lies in the neural canal pass out in the intervals between adjacent neural arches. Instead of this, however, they sometimes perforate the neural arch.

Neural spines, though generally single, may be double or altogether absent, and sometimes, as in Tortoises, they may intimately coalesce with superimposed dermal plates.

Cervical Vertebrae.—As has been already indicated, no vertebrae can be distinguished as cervical in the class of Fishes. Never-theless the first three or four vertebrae next the head may, in some of these animals, present a marked difference from the succeeding vertebrae, being much elongated and united to each other by suture, as in Fistularia and Bagrus, and they may, as in the latter Fish, develop a continuous inferior vascular canal. The second and third vertebrae may form a hollow bladder-like case of bone, as in Cobitis, or send outwards or downwards special processes, as in the Carp.

In Amphibians only a single vertebra can be called cervical, but in Sauropsidans the number may be very large. Thus in the Swan it amounts to twenty-five, while in some of the Plesiosaurians it exceeded forty. Birds, being animals which have to perform with the beak functions which in most animals are performed by limbs, require to have a very movable neck ; and consequently a considerable number of joints (and therefore of vertebrae) are required in the neck, which is the only part of the spinal column that is very flexible. In Serpents, which have the whole spinal column very flexible, no really satisfactory line can be drawn between cervical and dorsal vertebrae. In Lizards there are usually from seven to nine, but in the whole class of Mammals (whether the neck be very long, as in the Giraffe, or, like that of the Porpoise, extremely short) there are constantly but seven cervical vertebrae, except in the Sloths, which may have from nine to six, the Manatee, which has but six, and the Manis, which may have eight. All the cervical vertebrae may become anchylosed together into a single mass, as usually in the true "Whales. Ordinarily in Mammals the transverse process is said to be perforated, i.e., there are two such on each side, which are short and connected at their distal ends by a bony bridge which represents what, in the thorax, is known as a rib, as is shown by their condition in other classes of "Vertebrates. Indeed in the lowest Mammals (Echidna and Or-nithorhywshus) these osseous bridges have the form of distinct, more or less Y-shaped bones, as also in the Crocodile, where they are much prolonged. In many Lizards and Birds the posterior cervical vertebrae bear long ribs, and are only counted as cervical because such ribs do not reach the breast bone, while more pos-teriorly placed ribs do attain it. The two superimposed transverse processes, with the rib joining them attached to succeeding vertebrae, form on each side of the neck a sort of bony canal in which runs the vertebral artery. Sometimes, however, as in the Camels and Llamas, this canal is replaced by one excavated in the neural arches. In some Cetaceans the external bar (or rudimentary rib) is wanting, so that there come to be two elongated transverse pro-cesses on each side.

Successive cervical vertebrae may differ strikingly one from another. Thus in the common European Terrapin we find the fourth cervical vertebra with its centrum convex in front and concave behind. The centrum of the fifth is biconvex. That of the sixth is concave in front with a double convexity behind. The seventh is doubly convex both in front and behind. The eighth is doubly concave at each end. The ninth is doubly convex in front and singly so behind.

The first cervical vertebra is known as the atlas, and joins the skull, which in Man it supports. It may be fused in one solid mass with the skull, as in the Sturgeon, or with a certain number of vertebrae, as in the Bays. It may be united by suture, as in Bagrus. The vertebral part of the atlas may be unossified, as in the "Wombat, or remain a distinct bone, as in the Thylacine. The neural spine may be detached from the neural arch, as in the Crocodile and Tunny. Its ventral part may send out a pointed process towards the head, as in Amphiuma. It may develop two concave surfaces to articulate with the skull, as in Amphibians and Mammals, or only a single cup, as in Sauropsidans generally.

The second cervical vertebra is known as the axis, and is dis-tinguishable in all Vertebrates above the Ichthyopsida. Its centrum develops anteriorly a special peg-like or tooth-like pro-minence known as the odontoid process, round which the head and atlas vertebra turn as on a pivot. This process may (as in many Beptiles and in the Omithorhynchus amongst Mammals) remain a distinct bone, and is regarded as the true centrum of the atlas, which thus generally coalesces into the axis vertebra instead of with the other portions of its own vertebra. The odontoid process may be absent in certain Mammals, as amongst Cetaceans.

Dorso-Lumbar Vertebrae. — The vertebrae which come between the cervical vertebrae and those (sacral) which support the pelvic limbs, or, when these latter are absent, the vertebrae between the cervical and the caudal vertebrae, form the vertebrae of the trunk. These are subdivisible into dorsal and lumbar when some of them (always the more anterior) bear ribs and others do not but have transverse processes only.

The number of trunk (or dorso-lumbar) vertebrae varies greatly, being very few in Frogs and Tortoises and very numerous in Serpents. In Mammals it ranges from about seventeen, in some Primates, to twenty-seven, in Hyrax. A definite number of trunk vertebrae is characteristic of certain groups of Mammals, though this number may be made up by different numbers of dorsal and lumbar vertebrae.

Dorsal Vertebrae.—Rib-bearing vertebrae are structures constantly found in all Vertebrate animals save certain Fishes and Amphibians. Dorsal vertebrae must be considered as including the whole number of trunk vertebrae in Serpents, since in those animals the whole series of the latter support ribs.

An ordinary Mammalian dorsal vertebra consists of a body and neural arch with articular processes or zygapophyses and with a more or less elongated neural spine, and a transverse process which juts out and bears an articular surface at its end. This process answers to the more dorsal of each pair of transverse processes on each side of a cervical vertebra. Another articular surface placed at about the junction of the neural arch and centrum answers to the more ventral of each pair of transverse processes on each side of a cervical vertebra.

The rib which on each side of the vertebra articulates with these two surfaces has generally itself such a surface at its prox-imal end (or head) and another on a more or less marked promi-nence called the tubercle of the rib. These are respectively designated the capitulum and tuberculum, and therefore the pro-cesses or articular surfaces of the vertebra to which the capitulum and tuberculum are respectively attached are called the capitular and tubercular processes or surfaces, as the case may be.

Sometimes each vertebra carries but one such articular surface (that for the capitulum of the ribs), as in the Dolphin. The two articular surfaces may co-exist at different levels on one single process, as in the dorsal vertebrae of the Crocodile, or they may be in close apposition, and, as it were, fused together, as in Serpents. They may, however, be supported by two quite distinct processes —one dorsal, the other ventral,—as in Ichthyosaurus and Meno-branchus.

Man has twelve dorsal vertebrae. This is a little below the average of his class, where there may be twenty-four, as in the Two-toed Sloth. There are more than twelve in most Reptiles, while in Birds there are mostly but seven to nine, or, very rarely, eleven, while there may, as in Ciconia alba, be but three reckoned as dorsal on account of the great extent of ossification in the sacrum or part connected with the legs.

The most remarkable modification of dorsal vertebres is that in Tortoises and Turtles, where the neural spines expand at their summits into wide plates which articulate by suture with each other and with similarly expanded ribs, to form the carapace.

In Serpents and Iguanas we have a special mode of vertebral interarticulation, over and above that formed by the zygapophyses. The neural arch develops a median anterior prominence with two articular surfaces called the zygosphene, and this fits into a corre-sponding median posterior recess called the zygantrum.

The maximum of complication as regards the interarticulation of dorsal vertebrae is found in the last dorsal of the Great Anteater. There each posterior zygapophysis develops two additional articular surfaces, one on each side of a notch which receives a process from the anterior side of the neural arch of the succeeding vertebra, which process is furnished with two corresponding surfaces. More or less distinct traces of certain additional processes, called met-apophyses and anapophyses, are sometimes present, but these it will be better to notice when describing the lumbar vertebrae, wherein they are more developed.

"We find in some Serpents peculiar processes which project down-wards and forwards from the base of the inner side of the transverse processes. We may also find present a long median inferior pro-cess extending vertically from the ventral surface of the centrum and as long as, or longer than, the neural spine of the same vertebra. Such processes are present in many Serpents—especially the poisonous ones—and in such Birds as the Penguin and Cormorant.

Lumbar Vertebrae,.—These are vertebrae interposed between the dorsal vertebrae and the sacrum; they are generally the largest vertebrae of each vertebral column, but sometimes (as in Bats and Pterodactyles) the cervical vertebrae are yet larger. Lumbar vertebrae are generally to be distinguished in Mammals, in Croco-diles, and in certain Lizards, but not in any Ichthyopsidan.

In Birds lumbar vertebrae are present, but are disguised and hidden by the extent to which the sacral ossification extends for-wards.

There are five lumbar vertebrae in Man, but the number in him is below the average of his class, though some Apes have but four. The Slow Lemur may have nine, the Two-toed Sloth has but three, and the Monotremes but two. These vertebrae are very numerous in the Oetacea, but the hinder limit of the lumbar region is more or less difficult to determine in these animals. The transverse processes are generally much longer than those of the dorsal vertebrae, and do not bear either capitular or tubercular arti-cular surfaces.

The processes already spoken of as metapophyses and anapophyses are generally much more developed in the lumbar than in the dorsal vertebrae. The former project forwards from the vicinity of the anterior zygapophyses, and the latter project backwards at a lower level. Both processes are to be detected in the last dorsal and first lumbar vertebrae of Man, but are at their maximum in the Armadillos. In addition, also, to the complexity of articula-tion before described as existing on the last dorsal vertebra of the Great Anteater, we find in that animal's lumbar region an addi-tional articular surface on each side of each transverse process.

The lumbar vertebrae may be anchylosed together and to other parts of the skeleton, as is the case in Birds.

Sacral Vertebrae.—These are distinguished from others, not only by their connexion with the skeleton of the pelvic limbs, but also by their coalescence and a certain degradation in their structure as compared with the trunk and cervical vertebrae. In Man five vertebrae thus coalesce to form the more or less triangular single bone known as the sacrum, but which always shows plain traces of its composite nature. Such coalescence and degradation generally exist in Vertebrates above the Ichthyopsida, which possess fully developed limbs. The coalescence of vertebrae is generally less extensive than in Man, though sometimes—as in Birds, some Edentates, and some Reptiles—it is much greater. The sacrum may be composed, of as many as ten vertebrae (as in some Arma-dillos) or of twenty (as in the Ostrich), and the lumbar or caudal vertebrae or both contribute to its formation.

In most if not all Mammals the sacral vertebrae—or the more anterior of them—have what are at first distinctly ossified elements in their transverse processes, which elements (like parts before noticed in the cervical vertebrae) are costal in their nature, i.e., represent rudimentary ribs, and in Crocodiles and Tailed Amphibians the sacral vertebrae have a distinct rudimentary rib attached to each transverse process. In Birds, however, the vertebrae of the sacrum, which have expanded transverse processes, do not develop these from distinct ossifications.

As regards the extent of connexion between the sacrum and the hip bones, union is more extensive in Man than in most Beasts, or in animals below Birds. Often in Mammals and almost always in Tailed Batrachians it may be confined to a single vertebra ; but ten vertebrae may be involved in this union in Mammals and twenty in Birds.

That the development of the sacrum is not always in proportion to that of the pelvic limbs is proved by the little Lizard Seps, in which, in spite of the rudimentary condition of the limbs, there are true sacral vertebrae.

No Fishes have a true sacrum, though, very rarely, as in the Tur-bot, we meet with a kind of false sacrum, formed by the anchylosis of the bodies and ventral spines of the first two caudal vertebrae.

Caudal Vertebrae. _—The vertebrae of the tail may be as many as 270, as in some Sharks. Amongst Mammals 48 (Microgale longi-caudata) is the highest number. Man has usually rudimentary caudal vertebrae, completely or partially united so as to form a small conical bone called the coccyx. Its proximal end articulates with the sacrum by its centrum and two small zygapophyses. It has besides two rudimentary transverse processes and two processes representing piers of the absent neural arch. The other vertebrae are destitute of processes and consist but of smaller and smaller vertebral centra. Thus the last vertebra is the very opposite of the first (or atlas), being all centrum, while the atlas has no centrum at all. The coccyx usually becomes anchylosed to the sacrum about or after the middle of life. The caudal region is still more reduced in some Bats, where there may be but two such vertebrae.

In animals provided with numerous coccygeal vertebrae, such vertebrae may be provided with processes and articulations as complex as those of other spinal regions. Transverse processes may be largely developed at the tail root, but almost always thence backwards diminish in extent; sometimes, however, as in the Armadillo (Chlamydophorus), they may increase in size backwards from the tail root. Rarely (as, e.g., in MenobrancJius) caudal vertebrae may be furnished with two ribs supported by both tubercular and capitular processes. Inferior arches may exist in the form of detached " chevron bows " placed beneath the intervals of successive caudal vertebrae, especially towards the tail root. They may be represented by processes or by continuously ossified inferior arches, which may, as in the Flat Fishes, be very prolonged, extend-ing downwards from each centrum as much as the neural arch and spine extend upwards from it.

Birds have generally six or eight, but may have ten, caudal vertebrae, at the end of which is a so-called "ploughshare-bone," consisting of two or more vertebrae anchylosed together.

The caudal region of the Frog is formed in a very peculiar way. It never consists of distinct vertebrae at any time of life, but is formed by the ossification of the membrane which surrounds the notochord, to which two small neural arches become attached. This structure is called the urostyle.

In Fishes (as in the Perch and Stickleback) there may be a urostyle continuous with the centrum of the last vertebra. Such a urostyle, unlike that of the Frog, is very sharply bent upwards. It is very small and inconspicuous. In other Fishes the hinder part of the notochord may (as in the Salmon) remain unossified and only pro-tected by lateral bony plates, but it is still sharply bent upwards. In a few Fishes (as, e.g., Polypterus) the hinder end of the spinal column is not bent upwards. In other Fishes again (as in the Sturgeon and many Sharks) the hinder end of the vertebral column gradually tapers and gradually (not suddenly as in the Perch and Salmon) inclines upwards. In the forms in which the hinder end of the vertebral column bends upwards—whether gradually or suddenly—the arches and processes beneath its hinder end exceed in size those on the dorsal side of it, as also do the fin-rays attached to them. Thus it happens that the part of the caudal fin which is on the ventral side of the gradually or suddenly bent-up part of the spinal column more or less greatly exceeds in size the part on the dorsal side. In those Sea Fishes {e.g., the Sturgeon and many Sharks) in which the upward flexion is gradual and manifest, the ventral part of the caudal fin is evidently the larger, and such a tail is called lieterocercal. In Fishes in which the hinder end of the spinal column is suddenly bent up and of minute size, so that its real condition is disguised, the caudal fin appears symmetrical and as if the parts dorsal and ventral to the end of the spinal column were equal. Such a condition has been named homocercal. Those Fishes in which the spinal column ends without turning upwards, and in which the parts of the caudal fin dorsal and ventral to it are really and not only apparently symmetrical, are said to be diphycercal.

Sternum.—The breastbone or sternum extends more or less along the middle line of the ventral region of the anterior part of the trunk in all Vertebrates above Fishes, except Serpents and a few other Reptiles.

Almost always it is connected with the more anterior ribs. Its anterior end is distinguished as the manubrium or presternum, and its hinder is called the xiphoid process or xiphisternum—the middle part being the " body " or mesosternum. A sternum may exist without ribs, or without forming any cartilaginous or osseous connexion with ribs, as in the Amphibia. The plastron of Chelonians might well be supposed to be a great sternum, more especially as the plate-like ribs are connected with it. It appears, however, that this great complex plate does not really include a sternum.

The before-mentioned threefold division of the sternum is normal in Mammals, and also exists, though more obscurely, in Birds and Beptiles. Even in Mammals it is not universal; the manubrium only may be present, as in the Greenland Whale, or the manubrium and xiphisternum without any mesosternum, as in the Dugong.

In Tailed Amphibians and the Slow-Worm (Anguis) we have a single sternum, which may be mesosternum only, while in many Frogs and Toads we have only the latter and the xiphisternum. The manubrium may develop a median keel, as in Bats, the Mole, and Armadillos; or the mesosternum, as in the Tamandua; or the mesosternum and xiphisternum, as in most Birds.

The xiphisternum may assume various forms, but attains its maximum development in Birds, where it forms the part of the sternum posterior to the attachment of the ribs, and may consist of a median and four lateral processes, as in the Fowl. It bears the greater part of the keel.

In the Monotremes there is a median ossicle in front of the manubrium, which is often called the episternum. It is really a part of the appendicular skeleton.

Ribs.—Mammals possess a greater or less number of ribs, which are mostly long, slender, curved bones, extending downwards from the transverse processes or bodies of the vertebrae, the more anterior of them forming a junction with the sternum. The part of the skeleton formed by the rib-bearing vertebrae, the ribs, and the sternum is called the thorax. In Man (see ANATOMY) there are twelve ribs (on each side of the body), whereof the first seven join the sternum by the intervention of cartilages, and are called " true ribs." The other five, which do not join the sternum, are called " false ribs." Each rib (except the last two on each side) has a double attachment to the spinal column. At its proximal end it has a rounded " head " or " capitulum," which articulates with the capitular surface of a dorsal vertebra. At a little distance from the capitulum is another rounded articular prominence called the " tubercle " or " tuberculum," which joins a vertebral tubercular surface. The part of the rib between the head and the tubercle is called the "neck." At its distal end each rib has attached to it an elongated cartilage called " costal." Those costal cartilages which do not join the sternum either end freely or blend with the costal cartilage next in advance.

Frogs and Toads have no ribs, nor can they be said to exist in some Fishes (e.g., the Chimaera, the Seahorse, the Lamprey and its allies) ; but in the immense majority of Vertebrates there are cartilaginous or osseous ribs, attached by their proximal ends to the vertebral column, and tending to surround the trunk.

All rib-bearing animals have both "true" and "false" ribs, save Serpents, Fishes, and Chelonians, which can have no true ribs since they have no sternum, and Tailed Amphi-bians, in which, though there is a sternum, no ribs join it. There may, however, be but a single pair of true ribs—as in the Whalebone Whales. The ribs are exceptionally broad in the Two-toed Anteater, where they overlap one another. The number of ribs has already been indicated under the head of " dorsal vertebrae," though in Birds we may have short ribs attached to the cervical vertebrae, and others coming from vertebrae which are generally counted as " sacral." There may be as few as five or six pairs, as in Amphiuma ; or the numbers may reach 320, as in some Pythons. In many Beptiles, as in the Crocodiles, there may be cervical ribs; and there may even be caudal ribs, as in Menobranchus.

The function of aiding respiration is one which the ribs possess in the higher Vertebrata, but quite other purposes may be subserved by them in addition to, or instead of, respiratory action—namely, locomotion, change of form, or bodily protection as armour. Thus the ribs may form a solid case for the safe keeping of the parts within, co-oper-ating in this office with other skeletal structures so as to form the " carapace " of Tortoises already noticed. Bibs may be the main agents in locomotion, as in Snakes, which glide along by the successive application to the ground of the edges of their ventral scales, which is brought about by the motions of the ribs, the ends of which are connected with the inner surface of such scales. In the little flying Lizard Draco certain much elongated ribs serve to support a parachute-like flying membrane, and in the Cobra it is certain ribs which sustain its " hood " when distended.

The presence of a distinct "head" and " tubercle" is a general but not constant character, and the head of the ribs may be connected with two vertebrae or only with one vertebra. The ribs may bifurcate proximally into two equal diverging branches, one representing the "head" and the other the " tubercle." A small backwardly-projecting structure termed an " uncinate process " may be given off from the ribs and may ossify as a distinct bone, as in most Birds and in the Crocodile.

Sometimes (as in Monotremes and many Lizards) a third segment may be intercalated between a rib and its sternal cartilage, and sternal cartilages may be represented by bones, as in Birds and Armadillos. In some Lizards the sternal cartilages of opposite ribs are continuous in the mid ventral line. There may be no representative of a sternal rib, as in Fishes and Batrachians.

Barely, as in the Crocodile, there may be ventral rib-like structures in the wall of the abdomen, which meet and are attached ventrally, but are " free " at their dorsal ends. These cannot, however, be counted as true ribs. Fishes have often two series of ribs on each side of the body, and in Polypterus some vertebrae may have four ribs on either side. In Fishes the ribs may also be in part attached to the neural spines above or to the haemal spines below the vertebrae.

Cranial Skeleton.

By the cranial skeleton we mean the skull, or that pari of the axial skeleton which serves to shelter the brain (or anterior expanded end of the central part of the nervous system), together with solid structures continuous or more or less directly connected therewith. Such a structure exists in every Vertebrate animal, except the Amphioxus, which has no brain. Nothing of the kind is known to exist in any Ascidian or in any Invertebrate animal,— unless that cartilage of Cuttlefishes which serves as an investment of the nerve centres and a support for the optic and auditory organs may be deemed a true cranial skeleton, since its portions just enumerated make it, as we shall shortly see, very analogous to a true skull.

The cranial skeleton is, of course, at first composed entirely of soft mesoblastic tissue, parts of which always become cartilaginous and generally also osseous, while more or less of its structure may remain in the condition of mere membrane. The bones which generally, as just said, enter into its framework may arise directly in the membrane or may be preceded by cartilage which ossifies, a circumstance which divides the cranial bones into two categories—"membrane bones" and "cartilage bones."

The cranial skeleton of Vertebrates is made up of three sets of parts :—(1) parts devoted to enclosing and protecting the brain; this is the cranium proper; (2) parts sheltering the organs of sense situated in the head— namely the optic, auditory, and olfactory capsules ; these skeletal parts consist of the bones, cartilages, and mem-branes of the orbit, the internal ear, and the nose respec-tively, or the perioptic, periotic, and perirhinal bones and cartilages; (3) parts continuous or more or less directly connected with the cranium, and applied to aid nutrition in the form of deglutition or respiration; such skeletal parts are the jaws and arches (or parts of such) behind the jaws known as the hyoidean and branchial arches.

1. The Cartilaginous Cranial Skeleton.—This is formed "by a differentiation within the membranous cranium," and consists of two plates (parachordals) placed one on each side of the anterior part of the notochord, and forming with the latter the floor of the hinder part of the cranium, which part is known as the basilar plate. The carti-laginous auditory capsules are closely united to the outer sides of the basilar plate. From the anterior margin of that plate two bars, called the " trabecules," diverge for-wards from the anterior end of the notochord, and then approximate, so as to enclose what is known as the pituitary space, and also the floor of the anterior part of the cranium. Thence they advance (generally united) into the nasal or ethmoidal region of the skull, forming a median nasal septum, having a cartilaginous olfactory capsule on each side of it, and developing lateral pro-cesses in front of and behind those capsules. Only in the Cyclostomata is there a single olfactory capsule instead of a pair. The nature of the parachordals and trabecular is disputed, but opinion inclines to regard them as corresponding to the neural arches of the spinal skeleton,— except the part around the notochord, which corresponds with centra in an unsegmented condition.

Upgrowths arise on the outer side of each parachordal, and these meet above and thus form a complete dorsal arch in the hinder or occipital region of the skull. The posterior aperture of this arch is called the occipital fora-men, and through it the spinal cord enters the cranium, there to expand and become the brain. Lateral plates arise on each side farther forwards, in the anterior or sphenoidal region of the cranium. But these do not gene-rally ascend enough to unite together dorsally, at least they almost always form but an imperfect roof to the cranial cavity. This cranial aperture may be related to a median, dorsally placed, eye, which probably once existed in all Vertebrates, and still exists in a rudimentary condi-tion in many Lizards. The lateral plates grow together medianly in front, and more or less completely separate the cranial cavity from the ethmoidal region in front of it. Openings are left here and there in the cartilages of the cranial walls for the passage outwards of nerves from the central part of the nervous system; but these openings or foramina will be noticed in describing the osseous cranial skeleton. On each side of the sphenoidal region are the optic cartilaginous capsules, which, however, never become united (as do the others) with the cranium, and therefore are not generally reckoned as parts of the skull. A special median cartilaginous vertical upgrowth from the trabecular between these capsules may (as in Teleostean Fishes, Lizards, and Birds) form an interorbital plate beneath the most anterior part of the cranial cavity.

The third category of cranial skeletal parts is generally represented by a series of descending cartilaginous bars (or visceral arcs) on each side of the alimentary canal, running forwards beneath the cranium to terminate at the mouth.

As this lateral region of the head corresponds with the body wall behind it, and shows transitory indications of division (like the body wall behind it) into an inner part or splanchnopleure and an outer part or somatopleure, it is obvious that skeletal structures formed in its inner or outer part may be taken as belonging to different cate-gories. In the Cyclostomata, as in the Lamprey, we find cartilaginous bars placed in the somatic division exclu-sively,—bars which support and externally protect the series of gill-pouches on each side; and parts probably homologous with these somatic bars of the Lamprey are found also in some Sharks.





The Cyclostomes also possess complex labial cartilages which support the lips of their suctorial mouths. Representatives of these cartilages are also to be found about the mouths of many Fishes, as well as in the temporary suctorial mouth of the Tadpole; and they still persist in connexion with the olfactory capsules, though in a reduced form, in higher animals. The most important members of the third category of cranial skeletal parts are—(1) the series of cartilaginous arches lying in the splanchnic or inner region of the lateral wall of the head, which arches sup-port the gill-pouches on their inner sides and are known as the branchial arches, and (2) the arches seemingly in series with them, which are more anteriorly placed, and which are known as the hyoidean arches and the jaws.

One or other, or both, of these two sets of arches are well developed in all craniate Vertebrates, except the Cyclostomes, in which there are no true branchial arches, but only a hyoidean and a rudimentary jaw arch. There may be as many as seven branchial arches (e.g., in Noti-danus), but five are usually present in water-breathing Vertebrates. The hyoidean arch becomes segmented into two noteworthy portions, the upper of which is known as the hyomandibular portion.

The most anterior, or mandibular arch, also becomes segmented into an upper or metapterygoid portion, an inferior or Meckelian portion, and a median or pterygo-quadrate portion, which grows forwards in front of the metapterygoid portion, and forms the foundation of the upper jaw against which the lower jaw (formed from the Meckelian portion) bites.

The thus formed upper and lower jaws may come to be suspended from the cranium in one of three ways. (1) They may depend from the cranium directly, that is, with-out the intervention of the hyoidean arch; this arrange-ment is known as autostylic, and exists in all Vertebrates above Fishes, as well as in certain of the latter (Chimxra and the Dipnoi). (2) They may be suspended, by the co-operation of the hyomandibular portion of the hyoidean arch with their own metapterygoid portion ; this arrange-ment is known as amphistylic, and is found in Notidanus, Hexanchus, and Ostracion} (3) They may be suspended exclusively by the hyomandibular portion of the hyoidean arch (to the exclusion of their more proximal portion), as in most Fishes and the Skates—an arrangement known as hyostylic.

2. The Osseous Cranium.—The bony skull is formed partly by ossifications of the cartilage of the cartilaginous skull and partly by ossifications of the membranes investing or completing it. The cartilaginous cranium may, as in Elasmobranchs, be covered by a thin calcified layer with-out becoming ossified. It may, as in the Selachian Ganoids, remain itself quite unossified, and yet become enveloped by membrane bones. In most cases, however, the investment of the cartilaginous cranium by membrane bones is accompanied by a more or less complete ossifica-tion of the cartilage itself. In the Amphibia the carti-laginous cranium is to a not inconsiderable extent ossified, but the membrane bones which invest it are nevertheless easily separable from it. The most constant ossifications of the cartilaginous cranium are in the occipital region. In the Lepidosiren these are the only ones, a bone being thus formed on each side of the occipital foramen, which bones are known as the exoccipitals.

Many disputes have taken place as to what cranial bones (both cartilage and membrane bones) of one group of animals correspond with those of other groups. Such disputes still exist in certain cases, and it would be unwise to positively assert more than the existence of a general correspondence between the cranial bones of widely different Vertebrates—such, for example, as between Teleostean Fishes and Eeptiles or Mammals.

Beneath the occipital foramen the basioccipital bone arises, and it may, as in Birds and Eeptiles, develop a posterior prominence which joins with contiguous prominences of the exoccipitals to form a single " condyle" for articulation with the spine. On the other hand, there may be, as in Mammals and Amphibians, two lateral exoccipital condyles unaccompanied by any median basi-occipital prominence. In most Fishes we find only a concave articular surface behind the basioccipital, which thus resembles in form the vertebral centra, the anterior posterior surfaces of which are concave. A fourth bone, the supraoccipital, generally bounds the occipital foramen above.

In front of this occipital segment the auditory capsule, on each side, generally ossifies from three centres of ossification, which form the prootic, opisthotic, and epiotic bones respectively. Of these the first is the most constant, and is the only one which ossifies in the Frog. When all three are present, the prootic is anterior in position, the opisthotic inferior and posterior, and the epiotic posterior and superior. Sometimes, as in Fishes, two other supero-external bones may be formed in the auditory capsule, the more anterior of which is the sphenotic and the more posterior the pterotic.

The base of the cranium, in front of the basioccipital, generally ossifies as the basisphenoid, and a depression on its upper surface is known as the sella turcica or pituitary fossa. In front of the basisphenoid there may be, as in Mammals, another azygous bone, the presphenoid. The skull's lateral walls (in front of the auditory capsule) ossify as the alisphenoid and orbitosphenoid on each side, the latter forming the antero-lateral wall of the cranium. The optic capsule or sclerotic may be merely membranous, as in Mammals, or may ossify, as in Birds, but it never forms any solid connexion with the cranial walls.

The olfactory region very often ossifies as a median vertical bone (the mesethmoid) and two lateral ones (the lateral ethmoids or prefrontals). These ethmoidal ossifications may close the cranial cavity anteriorly, or may be altogether anterior to it. The olfactory and presphenoidal region may ossify very exceptionally as one bone. Such a condition we find in the Frog and its allies. These bones vary greatly in different classes of Vertebrates as to the degrees in which they anchylose together or remain distinct, and also as to the order in which those unite which ultimately coalesce. Similar differences occur with respect to the remaining skull bones. Speaking generally, we find the greatest amount of distinctness in the Osseous Fishes, and the greatest amount of coalescence in the class of Birds.

The membrane bones of the cranium are most conspicuous and constant on its roof. In Fishes we find every grade of transition between simple dermal scutes and true subdermal bones of the internal skeleton. Well-developed dermal cranial scutes are to be found in the Sturgeon and some Siluroids. Where the membrane bones still retain the character of dermal plates, those on the dorsal surface of the cranium are usually arranged in a series of longitudinal rows, continuing in the region of the head the rows of dermal scutes of the trunk. The dorsal cranial dermal bones differ in different Fishes as regards arrangement and number as well as size. Owing, how-ever, to their linear arrangements, they usually receive corresponding names, though it is very doubtful whether they can be considered as truly homologous. In most Bony Fishes, as in higher animals, we may generally dis-tinguish in the cranial roof one or two parietals, with an interparietal or upper (or upper part of a) supraoccipital behind the parietals, and a frontal or pair of frontals in front of them. A bone called the squamosal may also form part of the cranial roof, as in Mammals, and may send forwards and outwards a process which unites with another form, a preorbital bone, to form a zygomatic; arch. In front, above, behind, and beneath the orbit (in which lies the sclerotic) bones may arise termed malars and lachrymals, supraorbitals, and post-frontals respectively, and the zygomatic process of the squamosal may unite with a corresponding process from the malar or the post' frontal. The malar bone, or (as it is often called) the jugal, rather belongs, however, to the third category of cranial skeletal parts. The olfactory or ethmoidal region becomes roofed over in part by the frontals, in part by the lateral bones (belonging to the third category of cranial parts to be presently noticed) called the maxillae, but it is mainly roofed over by two bones (sometimes one bone) called nasals, which bound the posterior surface of the external nasal opening on each side of the skull. In Bony Fishes, Amphibians, and Serpents almost the whole cranium is invested below by a large membrane bone called the parasphenoid.

The nervous centres within the cranial cavity send forth nerves through certain definite small apertures or foramina, which show much constancy of position. As a rule, and in the highest class of Vertebrates, the olfactory nerves pass out medianly in front to the ethmoids, between the orbitosphenoids or the membranous parts which may represent them. The optic nerves perforate the orbitosphenoids, but may pass out behind them. In Lizards (e.g., Hatteria, Anguis, and many others) an aperture is left in the roof of the skull which is called the " parietal foramen." It serves for the reception of a third and rudi-mentary eye, the existence of which in Lizards was before referred to in noticing the cartilaginous cranium. It is a structure of great morphological interest. The nerves of the muscles of the eye, as well as the first of the three divisions of the fifth nerve, pass out in the interval between the orbito- and ali-sphenoids. The two other divisions of the fifth, as a rule, perforate the alisphenoid, the third the more constantly, the aperture for it being known as the foramen ovale, the less constant aperture for the second branch being called the foramen rotundum. The auditory nerve enters the auditory capsule (whether ossified or not) on its inner side, and does not pass out from it, but the facial nerve both perforates and traverses it. The glos-sopharyngeal, pneumogastric, and spinal accessory nerves pass out between the auditory capsule and the exoccipital, which latter bone is perforated and traversed by the hypo-glossal nerve.

Thus the osseous cranium (apart from the sense-cap-sules) consists of three arched segments: of these the hindmost is formed by the basi-, ex-, and supra-occipitals, the median by the basisphenoid, alisphenoids, and parietals, and the anterior by the presphenoid, the orbitosphenoid, and the frontals. These have been called "cranial vertebras," and certainly if the essence of vertebrae consists in their being a series of solid rings, fitted together and enclosing a tract of the nervous centres, then it must be admitted that the cranium—of the highest class of animals at least—is made up of three such vertebrae. Their development, however, is altogether different from that of true vertebrae, and no such resemblance to vertebrae is to be detected in the constituent parts of the cartilagin-ous cranium. Nevertheless it is undeniable that there is a singular secondary and induced resemblance to vertebrae in these ossified skeletal parts.

The osseous condition of the third category of cranial skeletal parts varies extremely in different classes of Vertebrates. The limits of this article are altogether insuffi-cient for more than a brief indication of the main varieties of the cranial structures of any of the three categories, and the reader must refer for details to the descriptions given in the various articles of this work which are devoted to different groups of animals.

The most anterior lateral descending bar or visceral arc is known as the mandibular arch. That part of it which extends forwards and forms the upper jaw presents'us with the following ossifications arranged in two rows— one external, the other internal. The external row, pro-ceeding from before backwards, consists of premaxilla, maxilla, jugal (or malar), and very often of a quadrato-jugal, which latter, when present, is generally in the form of a bar of bone (with an interval between it and the skull), forming, or helping to form, an inferior lateral external arch analogous to the superior lateral arch already noticed as the "zygoma." There may be a pair of pre-maxillae, or they may be represented by an azygous bone. The premaxilla, maxilla, and jugal often unite with the anterior outer margins of the nasal, frontal, and lachrymal to form a continuous bony external wall to the anterior part of the skull. The internal row of bones, proceeding again from before backwards, consists of the vomer, pala-tine, and pterygoid, which, with their fellows of the opposite side (and sometimes with the aid of the parasphenoid), form the bony roof to the month, which roof may (as in Mammals and Crocodiles) be a continuous bony partition, or may be but a sort of open bony framework. Besides the pterygoid proper, other ossifications, adjoining it, have been distinguished as the entopterygoid and ectopterygoid.

The lower part of the most anterior lateral visceral arc forms all or part of the lower jaw. In the Mammalia it forms the whole of that jaw, and is invested by but a single bone—the dentary. In other Vertebrates it forms but the distal, though greater, part of that jaw, and may be invested, not only by a dentary, but also by bones called angular, subangular, coronoid, and splenial. The jaw is further continued, proximally, by two bones—the articular and the quadrate—which are ossifications of the cartilaginous arc itself. This may, as in Birds and Bep-tiles, be directly articulated to the cranial wall, or it may be (as in Fishes) suspended therefrom by bones, the highest of which is termed the hyomandibular, which articulates with the ossified auditory capsule. The hyo-mandibular joins below two other bones, the anterior of which is called the metapterygoid and the posterior the symplectic, to both of which the quadrate is attached. Thus these four bones act as a " suspensorium" for the lower jaw, the joint between which jaw and the suspenso-rium is placed at the junction of the quadrate and the articular. In Mammals, parts answering to the suspen-sorium, the quadrate, and the articular form no part of the jaw but are of relatively minute size and are known as certain parts (the auditory ossicles, &c.) of the internal ear, and are protected externally by an ossifica-tion called the tympanic bone.

The second lateral descending bar or visceral arc, known as the hyoidean arch, may have its upper part ossified, in union with the preceding arch, as in the bony suspensorium of Fishes just described. On the other hand its upper part may, as in Mammals, be represented only by minute parts of the internal ear,—except the very summit of the arch, which forms the tympanohyal, and is anchylosed to the ossified auditory capsule of the internal ear. In Bony Fishes the hyoidean arch begins to free itself from the suspensorium, as a bone called the stylohyal, which is attached to the preceding or mandibular arch, between the hyomandibular and the symplectic. The arc then continues downwards as the epihyal and ceratohyal, ending below in the basihyal, from which a glossohyal may project forwards and a urohyal backwards. In Fishes certain styliform ossicles termed branchiostegal rays may project backwards from the hyoidean arch; and above them certain membrane bones called opercular bones—the oper-culum, preoperculum, suboperculum, and interoperculum —are attached above to the hyomandibular, and lie outside the mandibular and hyoidean arches.

In the air-breathing Vertebrates the hyoidean arch may be well developed or very imperfectly so, and concurs with parts belonging to the more posteriorly situated lateral arches to form a complex bone—the os hyoides—as will be further described.

These more posterior lateral arches—the branchial arches—attain their most complex osseous condition in Bony Fishes, which have commonly five of them, not solidly united to the skull above, but connected one with another inferiorly and with the inferior part of the hyoid arch. From below upwards these arches consist generally of a basibranchial, a hypobranchial, a ceratobranchial, an epibranchial, and a pharyngobranchial, but the hindmost arch is less fully and complexly formed.

In air-breathing Vertebrates the already-mentioned os hyoides consists of a central part or " body," to which are attached two pairs of single or jointed processes termed cornua. The anterior pair of cornua (known in human anatomy as the lesser cornua) represent the hyoidean arch, and may contain all its bones, including the " tympanohyal." The posterior pair of cornua (the greater cornua of human anatomy and the thyrohyals of Mammals generally) answer to or represent part of the branchial arches, and may be longer or shorter than the anterior pair of cornua. That they really have this homology is proved by the process of metamorphosis of the Tadpole, which in its early stage has distinct cartilaginous bran-chial arches that become the posterior cornua of the os hyoides of the adult Frog.

The osseous skull may, its bones remaining distinct, form a very solid whole, and the brain-case may be com-plete, as in Mammals, or it may be very loosely constructed and largely membranous, as, e.g., in most Lizards. Teeth may be connected with various bones,—most constantly with the dentary, maxilla?, and premaxillae,—but the palatines, pterygoids, parasphenoid (inPlethoclon), pharyngo-branchials, and even the basioccipital (Carp and Tench), may be dentigerous.

The structure of the skull is so exceedingly complex and varied that it is impossible within the limits of the present article to do more than give the above general indications. For further particulars the reader is referred to the anatomical details which will be found in the several articles of this work which are devoted to the description of different single groups of Vertebrate animals, and especially to the description of the skull of Man in the article ANATOMY.

APPENDICULAR SKELETON.

This part of the internal skeleton of Vertebrate animals normally supports two pairs of limbs only, but in one class—that of Fishes—there are azygous structures—the unpaired fins—which, as before said, must be reckoned as belonging to this category. These latter will be more conveniently treated of later. The whole appendicular skeleton may, however, be wanting, as in the Lamprey and in most Serpents.

The Skeleton of the Paired Limbs.-—The paired-limb skeleton normally consists of that of an anterior, pectoral, or thoracic pair of limbs and that of a posterior or pelvic pair. In certain species there may be but a single pair of limbs, which may either be the pectoral pair, as, e.g., in the Amphibian genus Siren, or the pelvic pair, as in the Reptilian genera Pipes, Lialis, and Ophiodes.

Normally each pair consists of diverging appendages— the limb skeleton proper—attached to a solid structure embracing parts of the trunk, i.e., a limb-root or limb-girdle. A thoracic limb-girdle may exist in a well-de-veloped condition without any limbs attached to it—as in the Slow-Worm (Anguis), but there is never a well-developed pelvic girdle without a rudiment of a pelvic limb.

In all Vertebrates above Fishes the limbs are divisible into a main part of the limb—arm or leg,—with a distal part or extremity—" manus " (hand) or " pes " (foot). We sometimes find (as in Lialis, Python, and Balxna) a rudimentary development of the skeleton of the leg without any rudiment of a pes; but we never find any rudimentary development of an arm without any rudi-ments of a manus. In the paired limb, as we have seen, a limb-girdle may be present without any part of a limb, but no part of the limb skeleton is ever developed with-out any limb-girdle. Normally the two limb-girdles are attached in a solid manner to the axial skeleton, in dif-ferent modes.

Normally the pectoral girdle is only thus connected with the axial skeleton by its ventral part, or with the ventral part of that skeleton, i.e., with the sternum, while it ends freely above, being dorsally connected with the axial skeleton only by soft structures. In Fishes, however, it may abut by its dorsal extremities on each side against the neural region of the spinal column, as in Baia clavata, or be connected with the head by skeletal struc-tures, as in Bony Fishes, e.g., Perch, Cod, &c.,—having all the time no connexion with the spine by its ventral part.

The pelvic girdle, on the contrary, is normally connected by its dorsal part solidly with the axial skeleton, though, as in Fishes, it may not be at all so connected. It never, however, abuts ventrally against the axial skeleton as does the thoracic girdle.

Appendicular Skeleton of Vertebrates above Fishes.

The paired limbs of all animals above Fishes are formed on one type, and differ greatly from those of the last-mentioned class. It will be convenient to describe first the general condition of the limbs in Mammals, Birds, Reptiles, and Amphibians.

Both the thoracic and pelvic limbs of these animals are divided, as before said, into main parts (arm and leg) and extremity (manus and pes). Each main part is further subdivided into a proximal segment (upper arm and thigh) and a distal segment (fore-arm and lower part of the leg). Each extremity is subdivided into a root portion (" carpus " and "tarsus "), a middle portion ("metacarpus" and "metatarsus"), and a terminal portion known as the digits. Thus the skeleton, e.g., of the hand of Man is composed of—(1) the root part of the hand or the '' carpus " (made up of eight small bones) ; (2) the middle part of the hand or "metacarpus" (made up of five long bones enclosed in the flesh of the palm); and (3) that of the digits, i.e., of the thumb (or " pollex") and of the four fingers, while the great toe (or " hallux") and the four other toes are the " digits " of the pes.

The joints between the proximal and distal segments of the main part of each limb are the elbow and the knee, and these are turned mostly (as in ourselves) in opposite directions. Primitively, how-ever, in all animals and permanently in some [e.g., Tortoises), both these joints are so conditioned as to open inwards—the elbow and knee being both directed outwards—while the palm of the manus and the sole of the pes are also both inwards in the embryo, and in the adult are applied to the ground, the digits of each extremity being directed outwards. This is the position in which the correspondence in structure between the thoracic and pelvic limbs is most obvious, and in it the whole surface of the limbs, which (on account of the muscles there placed) is known as the "extensor" surface, is turned outwards, whereas that known as the ' flexor " surface is turned inwards, wdiile the pollex of the manus and the hallux of the pes are both in front of their respective extremities, This primitive condition is altered during the process of development of Man and most air-breathing Vertebrates, the knee becoming bent forwards and the elbow backwards, while the fore-arm is twisted by a movement called "pronation," so as to enable the flexor or palmar surface of the manus to be applied in a direction parallel to that of the flexor or plantar surface of the pes.

In Bats the thigh is turned backwards, so that the knee bends backwards like an elbow. Were it necessary in these animals to apply the sole of the pes to the ground with the digits forwards (as in most animals), then a pronation of the lower leg would be needed in them, similar to the pronation of the fore-arm, which, as above said, takes place in the majority of animals here referred to—air-breathing Vertebrates.

The Thoracic or Pectoral Limb-Girdle.—The shoulder-girdle normally consists of the following bones or cartilages :—(1) a superior portion, generally a more or less broad plate of bone, called the scapula, the upper part of which may remain cartilaginous and more or less distinguishable as a suprascapula ; (2) a posterior inferior portion, named the coracoid, which may or may not be continuous with the scapula, and may have additional parts or subdivisions distinguished as the coracoid proper, pre-coracoid, and epicoracoid ; at the junction of the scapula and coracoid there is a concave articular surface—the glenoid cavity, into which the pectoral limb is articulated; (3) an anterior inferior portion, called the clavicle, which may abut against an azygous median structure known as an interclavicle, the two being distinguished from the other elements of the girdle by being more or less entirely membrane bones.

These structures are found well developed in many Lizards and quite exceptionally in Monotremes amongst Mammals. In them and in Birds, the coracoids are largely developed, while they remain mere processes of the scapula in non-Monotrematous Mammals, and sometimes are quite rudimentary. In such Mammals the pectoral arch is only completed inferiorly by the clavicles which abut against the sternum, but sometimes (as, e.g., in Ungulates) are altogether absent. The " merrythought " of Birds is a clavicular structure. In Amphibians the two halves of the shoulder-girdle are each formed of a continuous plate. Some anatomists reckon part of this as representing a clavicle, but this determination is very doubtful.

The Pelvic Girdle.—This girdle, like the former one, normally consists of three parts—one dorsal, the ilium, and two ventral, whereof the more anterior is the pubis and the posterior the ischium, and all these are cartilage bones. The pubis generally meets ventrally its fellow of the opposite side, but not always so. The ischia meet ventrally more rarely. In Birds and certain extinct Reptiles a third element, the post-pubis, intervenes between the ischium (more or less parallel to the latter) and a pubis which may be fully or only rudimentarily developed. At the junction of the ilium and the ventral pelvic elements there is a concave articular surface for the pelvic limb, the acetabulum. An interval between the pubis and ischium of each side is known as the obturator foramen. We find amongst Amphibians there is a peculiar cartilage in the ventral median line in front of the pubis, which has been called the prepubic cartilage. In Marsupials and Monotremes a bone extends forwards in front of each pubis, and these bones are known as the marsupial bones.

The Limbs. —The general condition of these organs and the bones supporting them in Vertebrates above Fishes having already been indicated, it remains but to fill in a few details as to their normal structure and its principal varieties.

A. Pectoral.—The bone of the upper arm is called the humerus, and is more or less cylindrical in shape, with an expansion at each end. It may, however, be almost as broad as long, as in the Mole and some Cetácea. The lower arm is generally furnished with two bones, the radius and the ulna, placed side by side. The ulna may be more or less abortive, as in Ruminants and Bats, but it may be the larger of the two fore-arm bones, as is the case amongst Birds.

The carpus may have its parts more or less permanently cartilaginous, as in some Urodeles and Cetaceans.

Taking the carpus of Man as a type of the ossified carpus (for further details, see ANATOMY), it consists of the eight following short bones arranged in two transverse rows. The proximal row (that next the arm) includes the scaphoides, lunare, cuneiforme, and pisiforme, while the distal row (that next the fingers) comprises the trapezium, trapezoides, magnum, and unciforme—starting, in each enumeration, from the thumb side of the manus. The pisi-forme stands out from the rest, and is reckoned as a sesamoid bone or ossification of a tendon, rather than a true carpal ossicle. There may he an analogous sesamoid ossicle on the other side of the wrist (on the side of the scaphoid) even in Apes, and this obtains its maximum in the Mole, where it strengthens and broadens the manus for digging. The true carpal bones may be more numerous or less numerous than in Man. Thus there may be an ossicle— called intermedium or centrale—placed in the mid line between the two rows of carpals, and this may be double, as in Crypto-branchus and some Siberian Urodeles. The unciforme may also be represented by two bones, as amongst Chelonians ; the pisiforme is often absent, and also the trapezium. The bones of the distal row are the less constant in number and development, and they may coalesce with the metacarpals, as in the Chameleon. Their development is related to that of the digits with which they articulate. All the true proximal carpal ossicles may unite into one bone, as in Pteropus, and the whole carpus may be reduced to two distinct bones, as amongst Birds.

The metacarpus, when fully developed, consists of five rather long metacarpal bones, as in Man. There may, however, be but two, and these united into wdiat is called a " canon bone " (as in Sheep, Deer, &c.); or there may be but a single one, as in the Horse,— answering to Man's third metacarpal. They vary in relative size and proportion in different animals, but are most remarkable for their length and slenderness in the Bats, while they are much elongated in the Horse and most Ruminants.

As to the digits, there may be but a single one, as in the Horse, or two, as in Ruminants and the Marsupial known as Chmropus. There may be three, as in the Rhinoceros, the Proteus, and in Seps ; or there may be four. The digits are never certainly more than five (except by monstrosity), although in the Ichthyosaurus extra mar-ginal bones along the manus give at least the appearance of more.

"When a digit is wanting it is generally the pollex (thumb), as in Spider Monkeys, but it may be the fifth, as in Pterodactyles, or both fourth and fifth may bo wanting, as in Birds. The pollex may be more or less opposable to the others, as in Lemurs, most Monkeys, and in Man, or two digits may be opposed to the other three, as in the Chameleon.

The second digit may be greatly reduced, as in the Potto, or the third may be disproportionally slender, as in the Aye-Aye, or thick, as in the Great Armadillo. The digits may be enormously elongated, as in the Bats, or short, as in the Mole and the Land Tortoises. They may be very imperfectly developed, as in Birds. They may be so united by dense tissue as to be quite incapable of separate motion, as in the Cetaceans. The bones of the fingers are called phalanges, and there are always three of them to each digit except the pollex, which has but two in all Mammals with the exception of certain Cetaceans, which have more. There may be as many as fourteen phalanges in one digit in Globiocephalus. The proximal row of these bones may become anchylosed to the metacarpals, as in the Three-toed Sloth. In Reptiles the numbers of the phalanges often increase from two in the pollex to five in the fourth digit, as in the Monitor. The abortive manus of Birds has at its best but three digits, with two phalanges to the pollex, three to the index, and one or two to the third digit. The phalanges are very numerous in the Ichthyosaurus and Plesiosaurus.
B. Pelvic. —The bone of the thigh is called the femur, and is a long bone which varies less in its form and proportion in different animals than does the humerus. It is, however, relatively very short in the Seals, and still shorter in the Ichthyosaurus.

In front of the knee-joint there is generally present a large sesamoid bone known as the knee-pan or patella. This, however, may even in Mammals be very small, as in Bats and Seals, or wanting altogether, as in the "Wombat.

The leg below the knee is supported by two long bones, the tibia and the fibula, placed side by side, whereof the former is the more internally situated, the larger generally, and the more constant. The two bones may anchylose together at each end, as in the Armadillos, or they may do so only below or only above ; the two bones may be completely fused together, as in the Frog. The tibia may be the only long bone, through the small development of the fibula, as in Ruminants and the Horse. The fibula may be quite styliform, as in Birds, or it may be developed interiorly but be atrophied at its upper end, as in Bats. Itmaj' be represented only by a small ossification outside the lower end of the tibia, as in the Ox, and with this there may exist a styliform rudiment of its upper-part, as in the Elk.

The joint by which the foot moves on the leg is situated between the lower end of the leg bones and the tarsus in Mammals and Am-phibians. In Birds and Reptiles, however, this joint is placed in the tarsus, the proximal part of which is firmly connected with the leg, while its distal part is firmly connected with the metatarsus.

The tarsus of Man consists of seven irregularly shaped, more or less short bones. Of these the astragalus joins the tibia and has the os calcis beneath it and the naviculare in front of it, while the metatarsals are supported (from the great toe outwards) by the internal, middle, and external cuneiform bones and by the cuboides, which is connected with the fourth and fifth metatarsals.

The tarsus may have its parts more or less permanently cartila-ginous, as in some Urodeles. The number of its bones, or cartilages, may be as many as nine, as in the Salamander, or be reduced to three, as in Proteus, or perhaps to two, as in Ophiodes. Two tarsal bones (the os calcis and naviculare) may take the form of long bones, as in Galago and especially in Tarsius. These two bones and the astragalus may be represented by a single bone, as in many Lizards, or may early unite with the tibia, as in almost all Birds. The astragalus may be represented by two bones, as in Urodeles. It may have an extra ossicle annexed to it, as in the male Omithorhynchus and Echidna. Two extra ossicles may be attached to the tibial side of the foot, as in the true Porcupine (Cercolabes). The naviculare may anchylose with one of the distal tarsal bones, as in the Ox and Deer, where it unites with the cuboid. The distal bones are less constant than the others, and they may anchylose with the metatarsals, as in Birds, the Chameleon, and the Three-toed Sloth. The cuboid may be represented by two bones, as in certain Urodeles. The internal cuneiforme may be wanting, as in the Ox, or coalesce with the middle one, as in the Horse.

The metatarsus when fully developed consists of five rather long metatarsal bones, as in Man, and never of more. There may be but a single developed metatarsal, as in the Horse (the third) and Chmropus (the fourth), or two fused together, as in the Sheep, Deer, &c., or three fused together, as in the Jerboa, or four so fused, as in many Birds. There may be but two metatarsals well developed, as in the Hog, or three, as in the Rhinoceros, or four, as in the Dog. They are never enormously elongated like the metacarpals of Bats, but they may all be extremely short, as in Land Tortoises and the Ichthyosaurus.

The digits vary in number, as has just been indicated with respect to the metatarsal bones sustaining them.

When one digit is wanting it may be the fifth, as in Birds, or the hallux (first or great toe), as in the Hare. The third and fourth digits may be only functional ones, as in the Ostrich ; but the third may abort, leaving only the fourth, as in Chmropus, or the fourth, leaving only the third, as in the Horse. The fourth and fifth may be the only functional ones, as in the Kangaroo. The hallux may be opposable to the other digits, as in Monkeys, Lemurs, Opossums, and Phalangers; or the first and fourth digits may be opposed to the second and third, as in Parrots ; or the first and second to the third, fourth, and fifth, as in the Chameleon.

The phalanges of the digits are in Man's whole class always three to each digit except the hallux, which (like the pollex) has but two _—save in the Orang, where it may have but one phalanx. They may be much more numerous than in Mammals, as in the pes of the Ichthyosaurus and Plesiosaurus. The numbers of the phalanges as we proceed from the first to the fifth digit may be 2, 3, 4, 5, 4, as in Lizards generally, or 1, 2, 3, 3, 2, as in the Salamander, or 2, 2, 3, 4, 3, as in the Frog. In Birds (where the fifth digit is more developed) the numbers of the phalanges, proceeding from the hallux, are mostly 2, 3, 4, 5 ; but they may be 2, 3, 3, 3, as in the Swifts, or 2, 3, 4, 3, as in the Goatsuckers.
Appendicular Skeleton of Fishes.

The Paired Limbs.—Most Fishes possess two pairs of limbs, known as the pectoral and ventral fins, which respectively cor-respond to the pectoral and pelvic limbs of higher Vertebrates. These limbs are attached to corresponding limb-girdles, whereof the pelvic girdle is always inferior in development and never attains the large relative proportions and fixed position of the pelvic girdle of non-Piscine Vertebrates.

Very often, however, the ventral fins are entirely wanting, and the pectoral fins are sometimes wanting also. In the latter case there is usually present more or less of a pectoral limb-girdle, though it may be, as in Murmnophis, little more than a filament. In all non-Piscine Vertebrates the right and left limbs are symmetrically and equally developed, but in the Flat Fishes (Pleuronectidm) one pectoral fin may be larger than the other, or one may disappear, as in Monochimis.

The situation of the paired limbs is, in Elasmobranchs, Ganoids, and a good many Teleosteans, similar to that they hold in higher Vertebrates, but in some other Teleosteans (such as the Fishes on that account called "thoracic") the ventral fins are placed far for-wards so as to come immediately behind the pectoral fins, while in yet other Teleosteans (known on that account as " jugular " Fishes) they are placed even in front of the pectoral fins.

The pectoral girdle may consist of a simple cartilaginous arch, as in Elasmobranchs, or it may be composed, as amongst Teleosteans, of two bones meeting ventrally, each being commonly regarded as a clavicle which is continued up dorsally to the skull by the inter-vention of a supraclaviele and a post-temporal. Besides these there is a cartilaginous element on each side which usually ossifies in two pieces, the upper one of which is reckoned as representing a scapula and the lower one a coracoid. These parts are annexed to the inner side of the clavicle, where also there is sometimes found a styliform bone, more dorsally placed, called the post-clavicle.

The pelvic girdle is represented in Elasmobranchs by a transverse cartilaginous structure formed of two separated or two medianly-united pieces, each, of which sometimes, as in Chimera and Callo-rhynchus, shows much resemblance to the innominate bone of higher Vertebrates in that it sends up a process simulating (and probably representing) the iliac element and possesses a sort of obturator foramen. In Osseous Fishes the pelvic girdle is normally repre-sented by two innominate bones medianly joined, each of which may, by rare exception, as in Lophius, send up a tall ilium-like process. In no Fish, however, does the pelvis become solidly united with the spinal column. In the cartilaginous Ganoids it is very rudimentary, and each lateral portion (which has a slightly-developed pubic and iliac process) is separated from its fellow on the opposite side, while in Lcpidosiren there is only a single simple median cartilage with no iliac process.

The skeleton of the pectoral limb, or fin, of most Elasmobranchs consists of three considerable basal cartilages, placed side by side, articulating with the pectoral arch, and named the propterygium, the mesopterygiuni, and the metapterygium. Of these the proptery-gium is proximal or anterior in position. To the distal ends of these are articulated a number of slender elongated more or less segmented radial cartilages, and to the distal portions of these are annexed the horny fin-rays which form the solid supports of the distal portion of the fin.

Sometimes there may be but two and rarely only one basal cartilage, which one must then be considered as representing the whole three condifferentiated. In Ceratodus there is a single basal cartilage followed by a series of small cartilages—secondary radial cartilages diverging from both sides of that series and having fin-rays annexed to them. In Lcpidosiren the limb skeleton is still more simplified, consisting only of a single series of short slender cartilages with small fin-rays attached to one side alone, without the intervention of any radial cartilages.

In some Bony Fishes (e.g., Polypterus) the basal cartilages are more or less ossified, as are also most of the radials next them, while to these small cartilaginous radials are annexed, which support ossified fin-rays. In some other Ganoids certain of the radial cartilages articulate directly with the pectoral arch. In the Teleostei a few, not above five, more or less ossified cartilages lie side by side and articulate with the pectoral arch, and one or more rows of small cartilages succeed to them. These two elements represent the basal and radial cartilages of Elasmobranchs, and to them are articulated the relatively large fin-rays which make up the far greater part of the Teleostean pectoral limbs.

The skeleton of the ventral fin or pelvic limb is almost always more simple than that of the pectoral one. Only very rarely, as in Ceratodus, Lcpidosiren, and Callorhynchus antarcticus (see Trans. Zool. Soc, vol. x. p. 455, and plate lxxix. figs. 3 and 4), have they a close, or pretty close, resemblance. Generally the Elasmobranch ventral limb is supported by an elongated cartilage, the basiptery-gium, which articulates with the pelvic cartilages and bears on its ventral border a series of cartilaginous radialia with which the fin-rays are connected. In Polyodon folium there are only radials which support fin-rays but are not themselves supported by any basipterygium, nor is there any pelvic cartilage. In the Teleostei the fin-rays are directly attached to the osseous pelvic elements.

The Unpaired Appendicular Elements. —Besides the two pairs of limbs there are, as has been mentioned, certain azygous structures commonly known as the unpaired or azygous fins or limbs. They are only found in Fishes, and consist of the dorsal, caudal, and anal fins. These may all run one into the other and form a continuous fin fringe to the body from the head round the tail and forward again to the vent, as in Eels and many Gadoid and Blennioid Fishes. In most cases, however, there are one or two distinct dorsal fins, and an anal fin also distinct from the caudal one.

The structure of the dorsal fin in Elasmobranchs is singularly like that of their paired fins, inasmuch as it is supported by an elongated or segmented basal cartilage or cartilages, from the dorsal margin of which radial cartilages (generally elongated, slender, and seg-mented) proceed, having the fin-rays connected with them distally. The basal cartilages may or may not be directly connected or become confluent with the subjacent spinal skeleton. There may be (as in the second dorsal of Callorhynchus antarcticus) but a single longi-tudinal series of more or less elongated cartilages side by side, like radial cartilages devoid of any subjacent basal cartilages. In the Teleostei the fin-rays may be osseous and in the form of more or less strong spines, or soft and of a horn-like consistency, and segmented both vertically and horizontally; and fin-rays generally consist of two (right and left) halves, which, although closely applied together for the greater part of their length, diverge proximally to embrace the skeletal element to which they are annexed. These latter elements in the Teleostei are small ossicles or chondrifications, termed " inter-spinous bones or cartilages." They extend upwards between the neural spines of the axial skeleton and the dorsal surface of the body.

Anal fins are essentially similar in composition to dorsal fins.

The caudal fin is modified according to the condition of the posterior termination of the axial skeleton, the different condition of which in Fishes has already been noticed (p. 112). Much-modified axial elements generally form the support of the fin-rays, but the numerous complex and varied conditions which these parts may pre-sent in different forms is a matter of ichthyology, which can hardly find a place in a general description of the Vertebrate skeleton.

Nature and Origin of Appendicular Skeletal Parts.—_ From the researches of the late Prof. Balfour it appears that the paired limbs arise as differentiations of continuous lateral folds or projections from the surface of the body, and the azygous fins arise as differentiated projections from its dorsal and ventral surfaces. Thus all these appendicular parts may be viewed as different species of one funda-mental set of parts (pterygia), for the sum total of which the term " sympterygium " has been proposed (see Trans. Zool. Soc, vol. x. pp. 481, 482). The paired limbs and azygous fins are of similar origin and nature. Separate narrow solid supports, in longitudinal series, and with their long axes directed more or less at right angles with the long axis of the body, were developed in varying extent in all these four folds or projections. These sup-ports have, it would appear, very often united to form basal cartilages, the original single and united condition persisting in such forms as the ventral fin of Polyodon and the second dorsal of Callorhynchus, both already noticed.

The paired limbs are thus, in all probability, essentially peripheral structures which have become more or less closely connected with the axial skeleton. Their proximal parts uniting and growing inwards have often become directly connected with parts of the axial skeleton. Thus the limb-girdles seem to have arisen,—namely, as ingrowths from the basal cartilages of the limbs; and therefore the whole appendicular skeleton belongs to a different skeletal category from that of the head and spinal column or axiai endoskeleton.

Bibliography.—Meckel, Traite General d'Anat. Comp., transí, from the German, Paris, 1829 ; Cuvier, Lecons d'Anat. Comp., 2d ed., Paris, 1837, and Recherches sur les Ossemens Fossiles, 4th ed., Paris, 1834; De Blainville, Ostéographie du Squeletle el du Systeme Dentaire des Cinq Classes d'Animaux Vertebres, Paris, 1839; R. Owen, Archetype and Homologies of the Vertebrate Skeleton, 1848, On the Nature of Limbs, 1849, and The Anatomy of Vertebrates, 1866 ; T. H. Huxley, Manual of the Anatomy of Vertebrate Animals, 1871, " On Ceratodus forsteri," in Proc. Zool. Soc, 1876 and "Pelvis of Mammals," in Proc Roy. Soc, vol. xxviii., 1879; C. Gegenbaur, Elements of Comparative Anatomy, Eng. transí., 1878, Untersuchungen zur Vergl. Anat.(Carpus und Tarsus), SchuUergürtel der Wirbelthiere u. Brustjlosses der Fische, and articles in the Jenaische Zeitschr., vols. v. and vii., and in Morphologisch.es Jahrb., vol. ii., 1876; W. H. Flower, Osteology of the Mammalia, 1870; A Gbtte, "Beitrage z. vergl. Morphol. d. Skeletsystems d. Wirbelthiere," in Archiv f. Mikr. Anat., 1877 ; W. K. Parker, Monograph on the Shoulder-Girdle and Sternum, Kay Soc., 1868 ; F. Balfour, A Treatise on Comparative Embryology, 2 vols., 1880-1881 ; J. K. Thacker, " Median and Paired Fins and Fins of Ganoids," in Trans. Connecticut Acad., vols. iii. and iv.; St George Mivart, Lessons in Elementary Anatomy, 1873, " On the Vertebrate Skeleton," in Trans. Linn. Soc, vol. xxvii., 1871, and "On the Fins of Elasmobranchs," in Trans Zool. Soc, vol. x., 1879. (ST G. M.)



Footnotes

"Homologous parts,1' or "homologues," are parts of an organism which cor-respond in relative position, that is, in their relation to surrounding structures, whether or not they serve the same ends. They thus differ from " analogous parts," which are parts performing similar functions whether or not they agree as to their relations of position to surrounding structures. Thus, e.g., the nail of a man's middle toe and the hind hoof of a horse are " homologous parts," but the hoot, as the support of the body and agent in locomotion, is analogous to the whole foot of a man.
Certain Ccelenterate animals consist but of two layers.
See F. Balfour's Comparative Embryology, vol, 1. p. 103.
* Op. cit., vol. i. p. 122, and vol. ii. p. 285.

Balfour's Comparative Embryology, vol. ii. p. 448.

Comparative Embryology, vol. ii. p. 449.
Owen's Anatomy of Vertebrates, vol. i. p. 31.

Balfour, ii. p. 466.
See Nature of May 13, 1886, p. 33.
Balfour, toe. tit., p. 490.
These terms were proposed by Professor Huxley. s Balfour, loc. cit., p. 475.

Balfour, loc. cit., p. 486.

The exact and precise homologies of these parts seem still to be siib judice.

Balfour, loc. cit., p. 499.
Balfour, loc. cit., p. 499.