Investigations into the histology of the nervous system in Vertebrata, have hitherto been chiefly confined to that of the Mammalia, or if applied to the lower members of that sub-kingdom, have not been undertaken so much with reference to the morphological relationship of animals, as to their bearings on physiology and the art of medicine. It may be stated in general terms that those who have worked at the morphology of the nervous system have not paid much attention to its histology; and, per contra, those who have investigated the histology have neglected its morphological bearing. Stieda, however, has investigated the brain in both aspects of the question, and Lockhart Clarke has made some reference to the nervous system of the lower Vertebrata in his classical investigations, but this bears but a subordinate proportion to the whole. Fritsch has recently published a treatise on the histology of the central nervous system in Fishes, in which the homology of the various parts is especially considered.