The above article was written by William Benjamin Carpenter, C.B., M.D., LL.D., F.R.S.; Fullerian Professor of Physiology at the Royal Institution; Professor of Medical Jurisprudence at University College, 1849; Examiner and Registrar of University of London, 1856; editor of the British and Foreign Medico-Chirurgical Review and a Popular Cyclopaedia of Science; author of Principles of General and Comparative Physiology and The Microscope and its Revelations.