478 PKOF. G. ELLIOT SMITH OX THE MOKPHOLOGT OF THE 



hemisphere as partly liomologous with the gyrus fornicatus, i. e. with a cortical area of the mammalian 

 brain which is not hippocampal. And yet both of these writers spoke with complete approval of 

 the view that the dorsal commissure, which springs from this mesial wall, is the psalterium or 

 hippocampal commissure. But if the mesial wall be partly gyrus fornicatus, its commissure cannot 

 be the psalterium, but must be the corpus callosum. There can be no doubt that the homology of 

 the mesial wall of the Reptilian hemisphere with the hippocampus is correct ; and the comparison 

 with the Monotreme brain, which I instituted in an earlier ])art of this memoir, shows conclusively that 

 no part of the mesial wall of the Reptilian hemisphere can be regarded as the homologue of the gyrus 

 fornicatus. 



It is strange that when Edinger held the mesial cortex of the Reptilian hemisphere to be 

 hippocampus, he called its commissure not " psalterium " but " corpus callosum " (oy;. cit. 1888); and 

 that when, eight years later, he changed his view and regarded it as including not only the 

 hippocampus but a uon-hippocampal pallial area, he called it not " corpus callosum " but " psalterium," 

 with a singular disregard for consistency. 



A comparison of the Reptilian cerebral hemisphere with that of the foetal and adult Monotreme 

 conclusively demonstrates that the whole of the mesial wall of the former which is ])laced above 

 the paraterminal body, together with a considerable part of the roof of the hemisphere, represents 

 unquestionably the mammalian hippocampus, and nuthing else. 



Although the Reptilian hippocampus is not so highly differentiated as its Mammalian homologue, 

 in that its margin is not yet definitely specialized to form a fascia dentata, I utterly disagree with 

 Kolliker in the statement that in the Reptilia " das Ammonshorn nicht gebiklet ist." ^ 



The arguments for the recognition of the medio-dorsal coitex in the Sauropsida as the hippocampus 

 may be concisely summed up in the following manner. A comparison of the brain of the Eutheria with 

 the Meta- and Protothcria shows that in both of the latter lowly orders of Mammals — widely as they 

 are undoubtedly separated the one from the other — the hippocampus has a much more extensive 

 distribution than in the formei'. Instead of being confined to the caudal portion of the hemisphere, it 

 extends forward as far as its morphologically anterior pole ; and as a search in the analogous region of 

 the Eutherian hemisphere has revealed vestiges of this cephalic extension of the hippocamjius, it is 

 natural to conclude tliat the ancestors of the Mam.malia possessed a hippocampus extending 



FORWARD upon THE JIESIALWALL OF THE HEMISPHERE TO ITS CEPHALIC POLE, «. C. TO THE NEIOHBOUR- 



HOOD OF THE OLiACTonY PEDUNCLE. The presence of a peculiarly distinctive histological formation in 

 the brain of the Reptile exhibiting not only the identical relations of the Prototherian cornu Ammonis, 

 but also possessing a structure which corresponds to one phase through which the Mammalian hippo- 

 campus passes in its ontogenetic development, can be rightly interpreted in only one way. These 

 reasons for thus homologiziiig the mesial cortex in the Reptile have hitherto never been categorically 

 formulated, and they constitute the only legitimate ground upon which such an interpretation can be 

 based. The early suggestions of Spitzka and Brill, and Edinger's extraordinary memoir of 1893, are 

 little more than guesswork. The value of the legitimate argument of Edinger in 1888 Avas utterly 

 destroyed by his later contributions in 1893 and 1896. And the sound work of Adolf Meyer left 

 the question in too undecided a state to be regarded as in any way settling the question. 



A hippocampus presenting the general features already briefly indicated exists in all classes of Reptilia, 

 but the details of its structure vary somewhat in the different families. lu all Reptiles the hippocampus 

 forms not only the whole of the mesial wall of the hemisphere above the paraterminal body, but also 

 part of the dorsal surface of the hemisphere. In many of the Chelonia, such, for example, as Emys 



* Much of Hie foregoing discussion has already been published in a short memoir entitled " Further Observations 

 upon the Fornix, with 8j)ecial Reference to the Brain of Si/dophilus," Journal of Anatomy and Physiology, vol. xxxii. 

 1898. It is repeated here in order to make the argument complete and intelligible. 



