THE NERVOUS SYSTEM. I9 1 



may be seen the anterior pyramids, composed of a band of 

 fibers lying on either side of the median line and apparently 

 emerging from the pons. They form a section of the 

 crossed 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 composed 

 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. 95). The central core of white matter forms 

 an arborescence known as the arbor vitcu. 



The cerebellum (Fig. 92) is seen to consist externally 

 of two lateral portions, the cerebellar hemispheres, a median 

 vermiform process and three pairs of peduncles or crura 

 which are bands of fibers uniting it with other parts of the 

 brain and cord (Fig. 96). The separation between the 

 hemispheres and vermis or vermiform process is most 

 marked on the cranial aspect. 



The peduncles are named, according to their location, 

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

 brachium pontis, is a dorsal prolongation of the fibers of 

 the pons Varolii (Fig. 93). In order to display the other 

 peduncles a portion of the cerebellum must be cut or picked 



