NERVOUS SYSTEM. VERTEBRATA. 161 



the hippocampus and do not form part of the fornix. But 

 as this series of neopallial fibres increases in number in 

 the Eutherian brain, the corpus callosum rapidly extends 

 and assumes the large dimensions which we usually associate 

 with this body. As it does so it naturally stretches a 

 portion of the great hippocampus (of which the fascia 

 dentata forms the only part visible upon the surface), and 

 the hippocampus atrophies in the region of stretching. 

 Thus in all mammals possessing a corpus callosum, the 

 hippocampus persists in an unchanged state only so far 

 forward (or so far up) as the splenium of the corpus 

 callosum, but its anterior part is represented by vestiges 

 upon the upper surface of the corpus callosum and along a 

 line joining the anterior extremity of the latter to the 

 olfactory peduncle. In man these vestiges together with 

 some longitudinal fibres in connection with them are 

 generally known as the longitudinal striae of Lancisius. 

 (For a fuller elucidation of these peculiarities see Journ. 

 Anat. & Phys. vol. xxxii. 1898, p. 30.) 



The precommissural area is also of interest in the 

 Marsupial, in view of the fact that in most other mammals 

 the upper part of the corresponding region becomes 

 stretched by the growing corpus callosum to form a folium 

 of the septum lucidum in each hemisphere. 



Upon the postero-superior region of the mesial wall of 

 the hemisphere, there is a short arcuate sulcus running 

 parallel to the hippocampal fissure. The corresponding 

 sulcus in the Ungulata was named the "fissura splenialis " 

 by Krueg, from its relationship to the splenium of the 

 corpus callosum. There can be little doubt that this sulcus 

 represents the anterior calcarine sulcus [i. e. the true 

 calcarine fissure, the stem of the Y-shaped complex] in 

 the human brain, and hence we may call it " calcarine " 

 in Sarcophilus (Elliot Smith, Proc. Anat. Society, 1899) *. 



* For the demonstration of this homology compare the conditions found 

 in Manis, Cholcepus, Daubentonia and the other Primates. 



The most diverse views are put forward by various writers as to the 



possibility of homologising the sulci of one brain with those of another. 



Some writers attempt to institute comparisons upon a purely topographical 



basis between the cerebral sulci of brains of even different Orders of 



VOL. II. M 



