The origin of the Corpus Callosum, a contrib. upon the Cerebral Comm. etc. 231 



(3) That the interval between the corpus callosum and anterior com- 

 missure is reduced so that these bundles are nearly in contact. 



(4) There is no septum lucidum. 



A general summary of the conclusions of recent authors upon 

 the cerebral commissures in the sub mammalian orders is as follows : 

 (!) The corpus callosum is wanting and the fornix, if at all, is 

 represented by a small commissure in the roof of the ventriculus 

 communis (Rabl-Kückiiakd, Bellonci). (2) The external olfactory 

 tract, a commissure of the olfactory lobes, the upper bundle in the 

 Amphibia and Sauropsida, represents the Pars olfactoria. (3) A com- 

 missure of the brain stem, the lower bundle, represents the Pars 

 temporalis, from the fishes upwards. (4) The internal olfactory tract 

 forms a chiasma, and this is sometimes true of the external olfactory 

 tract (Bellonci). 



Upon theoretical grounds the following objections arise against 

 this interpretation of the distribution and homologies of the cerebral 

 commissures. It places the course of the pars olfactoria, in the upper 

 internal portion of the mantle, and towards the corresponding region of 

 the olfactory lobes, a condition which is represented neither in the 

 embryonic nor adult position of this bundle in the mammalian brain 

 where its course is chiefly in the brain stem towards the lower external 

 area of the olfactory lobes. The pars temporalis, according to this 

 theory is a commissure of the brain stem, from its earliest develop- 

 ment, whereas in the mammalia, according to our present knowledge, 

 it belongs to the temporal region, which is a portion of the mantle. 

 The greater part of the brain mantle is thus left without any com- 

 missural supply, although in the lowest mammals, as we have seen, 

 the commissural supply extends to the inner wall and the greater 

 part of the lower and outer wall of the hemispheres. We thus have 

 to account for the origin of a new principle highly characteristic of the 

 mammalian brain viz. the union of corresponding portions of the 

 mantle of the two hemis})heres by commissural fibres. 



But theoretical considerations however plausible, do not constitute 

 com})arative anatomy, and the question really turns upon the actual 

 distribution of the fibres of the upper and lower bundles in the lower 

 vertebrata. It is upon this distribution that I venture to ground the 

 homologies presented in this paper. 



