THE BBAIN OF MAMMALS AND THE OLFACTOET APPAEATUS. 



311 



MoreoTer, the principal commissure of that portion of the mantle which 

 does not belong to the olfactory apparatus, the corpus callosum, is also so 

 small that it occupies a relatively small portion only of the mesial surface 

 of the hemisphere. Indeed, in some monotremes and in the marsupials 

 hitherto investigated a corpus callosum is entirely wanting {Symington, E. 

 Smith). 



Likewise, the fiber-systems passing down from the mantle, especially 

 those to the pons and the spinal cord, the fibers of the pes, are, in all verte- 

 brates very much less developed than in man. 



For the most part the olfactory apparatus is much more powerfully 

 developed than in man, but it may also atrophy very enormously, even 

 degenerate to such an extent that it almost disappears, as in the aquatic 

 mammals. Accordingly, mammals have been divided into osmatic and 



0^^m 



Fig. 140. — Brain of armadillo: Dasypus villosus. (Basal view.) 



anosmatic. The investigation of a very large series of animal brains shows 

 that the olfactory apparatus and the pallium develop entirely independently 

 of one another phylogenetically ; that the one may atrophy, the other attain 

 a higher degree of perfection; and vice versa. The greater perfection of the 

 olfactory apparatus manifests itself not only in the more powerful develop- 

 ment of the olfactory lobe and the olfactory region of the mantle, but also, 

 oh the other hand, in a very marked development of definite cell-groups and 

 fibers belonging to this apparatus in other parts of the brain. 



The olfactory apparatus will, therefore, be considered, first of all, as a 

 whole. Present in man, as relatively atrophied remnants only, it may be 

 more easily studied in many other mammals. 



Illustrations are here presented of the base of the brain of a calf and 

 of an armadillo. A large lobe is here seen which anteriorly is covered as if 

 with a cap by an enlargement, and posteriorly passes over somewhat directly 



