THE NERVOUS SYSTEM. 203 



pyramidal or motor tract connecting the brain with the 

 cord. The olivary eminence lies just laterad of the 

 pyramid. The fact that seven posterior cranial nerves 

 originate from the medulla shows the importance of this 

 part of the brain. The destruction of the respiratory 

 centers, or vital knot, lying ventrad to the caudal end 

 of the fourth ventricle causes instant death. 



The Cerebellum, or Little Brain.- -The cerebellum, 

 which in the cat lies caudad of the cerebrum and dorsad 

 of the medulla, resembles the cerebrum in being com- 

 posed of an outer layer of gray matter or cells and an 

 inner mass of white matter made up of fibers. While 

 within the cerebrum there are several important ganglia 

 or masses of gray matter, in the cerebellum only one 

 chief mass of gray matter, the corpus dentatum, is present 

 in each hemisphere. 



The outer layer of gray matter, known as the cortex, 

 is folded into numerous convolutions, between which are 

 deep sulci (Fig. 92). The central core of white matter 

 forms an arborescence known as the arbor mtcE. 



The cerebellum (Fig. 89) is seen to consist externally 

 of two lateral portions, the cerebellar hemispheres, a 

 median vermiform process and three pairs of peduncles 

 or bands of fibers uniting it with other parts of the 

 brain and cord (Fig. 93). The separation between the 

 hemispheres and vermis or vermiform process is most 

 marked on the cephalic aspect. 



The peduncles are named, according to their location, 

 superior, middle, and inferior. The middle peduncle, or 

 cms ad pontum, is a dorsal prolongation of the fibers of 

 the pons Varolii (Fig. 90). In order to display the other 

 peduncles a portion of the cerebellum must be cut or 

 picked away with the forceps. The superior one, known 

 also as the brachium conjunctivum, extends as a cord 



