Mammalia: Nervous System . l.'i 



are more or less elaborately folded in a manner resembling that of the 

 convolutions of the cerebral cortex. The middle lobe is called the 

 vermis because its conspicuous transverse convolutions cause it to 

 resemble a short, thick, segmented worm. The vermis is always well 

 developed, but the lateral lobes vary in size according to the degree of 

 development of the cerebral cortex. 



The cerebellum resembles the cerebral hemispheres not only in the 

 convoluted outer surface, but also in having a well-defined outer layer 

 of "gray substance," the cerebellar cortex. The demarcation between 

 this cortex and the underlying "white substance" is less regular than 

 in the cerebral hemispheres. On the surface of a sagittal section of the 

 vermis (Figs. 522, 528), the "white substance" is seen to be disposed 

 in the form of a treelike mass whose branches extend up into the 

 "gray" — hence the old name, arbor vitae, given to this arrangement 

 of the materials. 



The conspicuous feature of the ventral wall of the mammalian 

 metencephalon is a great transverse band of substance including some 

 "gray" masses ("nuclei" — clusters of nerve-cells) and great tracts of 

 libers. This transverse bridge, the pons Varolii (Figs. 522, 524B, 

 528, 530), connects the right and left cerebellar lobes. It makes a sharp 

 ventral demarcation between the metencephalon and medulla. 



The myelencephalon of mammals, compared to that of other 

 vertebrates, shows a minimum of change. More so than any other of 

 the five regions of the mammalian brain, it is truly palaeencephalic. 



CRANIAL NERVES 



The cranial nerves of mammals consist of the 12 pairs characteristic 

 of all Amniota (Figs. 521. 524, 530, 531). In their points of origin from 

 the brain and in their general plan of distribution, they differ in no 

 important way from the cranial nerves of reptiles. Certain peculiarities 

 of the three nerves of "special sense," I, II, and VIII, should be 

 mentioned. 



The olfactory (I) fibers coming from the olfactory epithelium of 

 each nasal cavity, instead of being bound up together in one compact 

 nerve, are disposed in the form of numerous small nerves. Each olfac- 

 tory lobe of the brain is close to the corresponding nasal cavity, being 

 separated from it only by the thickness of the mesethmoid bone. The 

 region of the bone adjacent to the two olfactory lobes is perforated by 

 numerous minute holes, through each of which passes one of the small 

 divisions of the nerve. The perforated part of the bone is called the 

 "cribriform (sievelike) plate." 



The optic nerves (II), emerging from the eyeball, converge to a 

 crossing or chiasma on the base of the diencephalon (Figs. 522, 528, 



