230 H- F- Osborn 



of the latter supply the ventral and outer and part of the upper wall 

 of the hemispheres. There can be little doubt that the development 

 of the anterior commissure is in a measure complementary to the that 

 of the corpus callosum. That this development is also in a measure 

 independent of the size of the olfactory lobes is shown b}^ the fact that 

 these bodies in some mammals, the hedgehog for example, are 

 larger than in some of the marsupials yet the anterior commissure 

 is smaller 1. In the Monotremes the corpus callosum is even smaller 

 than in the marsupials, and there is also a great reduction of the 

 septum (lamina terminalis), whereby the convex lower surface of the 

 corpus callosum is almost in contact with the upper part of the an- 

 terior commissure. 



Sander confirmed Flower's observations and in horizontal sec- 

 tions of the brain ofDidelphys showed that while there is a well 

 marked bundle of the anterior commissure which enters the olfactory 

 lobes (pars olfactoria), the greater part of the commissure spreads 

 out in the whole region of the frontal lobe which in the higher 

 mammals is supplied by the corpus callosum. The septum pellu- 

 cidum and fifth ventricle are absent (Macropus, Didelphys). 

 The forward division of the anterior commissure contains elements 

 which disappear in the higher mammals. 



Meynert (1871) observed that a further reduction of the Pars 

 olfactoria takes place pari -passu with the decrease in the size of 

 the olfactory lobes. 



Ganser (1878 pag. 279) observed that the Pars olfactoria in the 

 Rabbit's brain contained commissural fibres of the olfactory lobes 

 only; but this observation proved to have been founded upon an 

 imperfect experiment and in a second paper 2 he showed that this 

 bundle also supplies the anterior portion of the hemispheres. 



From the above we may deduce the following in regard to the 

 primitive condition of the cerebral commissures in the lower mam- 

 mals: (1) That the distribution of the corpus callosum is chiefly upon 

 the inner wall of the hemispheres, and that this bundle is as small 

 or smaller than the anterior commissure. (2) That the pars olfactoria 

 constitutes the principal part of the anterior commissure and supplies 

 a considerable area of the hemispheres as well as the olfactory lobes. 



1 This observation has been confirmed by Ganser (1878, pag. 298) who 

 does not however seora to have known of Flowers paper. 



2 I have not seen this paper, and learned this from Professor Gudden. 



