808 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



Further, in descending the vertebrate series, the lateral ventricles 

 become smaller in extent and of simpler form, the posterior cornna 

 being absent in all animals below the quadrumana, with the exception 

 of the seal. The gray matter of the hemispheres likewise diminishes, 

 until in fishes the gray cortex is so thin as to be almost indistinguish- 

 able to the naked eye. The corpus callosum has its highest develop- 

 ment in man, and in the lower mammals becomes both shorter from 

 before backward and thinner, and gradually becomes inclined upward 

 and backward. In the marsupials it is rudimentary, and in no vertebrate 

 lower than mammals is there any trace of the corpus callosum, it being 

 represented in birds, reptiles, amphibia, and fishes merely by transverse 

 commissural fibres crossing at the base of the cerebrum. As the hemi- 

 spheres become reduced in importance there is a corresponding simplifi- 

 cation in the medulla-oblongata and a diminution of the cerebellum and 

 pons, the pons being absent in reptiles, amphibia, and fishes. 



No olivary bodies or corpora dentata are to be distinguished in 

 animals lower than mammals, the anterior and posterior prominences and 

 restiform bodies often constituting the entire mass of the medulla 

 oblongata. 



The fourth ventricle, on the other hand, is more clearly marked in 

 the lower animals and is more directly continuous with the central canal 

 and spinal cord. 



Like the cerebrum, the cerebellum likewise diminishes in passing 

 from the highest to the lowest vertebrates. In birds the bulk of the 

 cerebellum is composed of the vermiform process, while in reptiles, 

 amphibia, and fishes this median portion is alone present. Like the 

 cerebrum, the cerebellum, also, becomes progressively simplified, the 

 laminae or convolutions diminish until they are comparatively few in the 

 bird, while they are absent in reptiles, amphibia, and fishes, in which the 

 surface is quite smooth. 



In the frog the cerebellum forms a simple, smooth band, and in the 

 lowest fishes is so reduced in size as to no longer cover the medulla 

 oblongata. 



In many carnivora and ruminants the cerebellum, instead of being 

 composed of broad, smooth, lateral hemispheres joined by the vermiform 

 processes, is very uneven on its surface, and apparently consists of a 

 cluster of a number of lobules. 



The internal structure of the cerebellum likewise becomes simplified 

 and the internal laminae of gray matter disappear. In general, it would 

 appear that the more complex the character of the movements of which 

 the animal is capable the higher will be the plane of development of the 

 cerebellum. 



Only in the large cetaceans and pachyderms is the brain absolutely 



