(8) Cyclops, Siren, etc.
The same feebleness of the formative energy (the Bildungstrieb of Blumenbach) which gives rise to some at least of the cases of undecided sex, leads also to imperfect separation of symmetrical parts. The most remarkable case of the kind is the Cyclops monster. At a point corresponding to the root of the nose there is found a single orbital cavity, sometimes of small size and the orbit and containing an eyeball more or less complete. In still other cases, which indicate the nature of the anomaly, the orbital cavity extends for some distance on each side4 of the milled line, and contains two eyeballs lying close together. The usual nose is wanting, but above the single orbital cavity there is often a nasal process on the forehand, with which nasal bones may be articulated, and cartilages joined to the latter; these form the framework of a short fleshy protuberance like a small proboscis. The lower jaw is sometimes wanting in cyclopeans; the cheek-bones are apt to be small, and the mouth a small round hole, or altogether absent; the rest of the body may be well developed. The key to the cyclopean condition is found in the state in of the brain. The olfactory nerves or lobes are usually described as absent, although Vrolik has found them in some instances; the brain is very imperfectly sac with thick walls, the longitudinal partition of dura mater (falx cerbri) being wanting, the surface almost unconvoluted, the corpus collosum deficient, the basal ganglia rudimentary or fused. The optic chiasma and nerves are usually replaced by a single mesial nerve, but sometimes the chiasma and pair of nerves are present.
The origin of this monstrosity dates back to an early period of development, to the time when the future hemispheres were being formed as protrusions from the anterior from the anterior cerebral vesicle or fore-brain; it may be conceived that, instead of two distinct buds from that vesicle, there was only a single outgrowth with imperfect traces of cleavage. That initial defect would carry with it naturally the undivided state of the cerebrum, and with the latter there would be the absence of olfactory lobes and of a nose, and a single eyeball placed where the nose should have been. A Cyclops has been known to live for several days. The monstrosity is not uncommon among the domestic animals, and is specially frequent in the pig. There is another congenital malformation, in which an eyeball is wanting from one of the sockets; but in that case is no defect of development in the bones, and the brain and nose are normal.
Another curious result of defective separation of symmetrical parts is the siren from foetus, in which the lower limbs occur as a single tapering prolongation of the trunk like the hinder part of a dolphin, at the end of which a foot (or both feet) may or may not be visible. The defects in the bones underlying this siren from are very various: in some cases is only one limb (thigh and leg-bones) in the middle line; in others all the bones of each limb are present in more or less rudimentary condition, but adhering at prominent points of the adjacent surfaces. The pelvis and pelvic viscera share in the abnormality. A much more common and harmless case of unseparated symmetrical parts is where the hand or foot has two, or more digits fused together. This syndactylous anomaly runs in families.
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Abnormal Development & Congenital Malformations - Table of Contents