INTRODUCTION 



retained, and the nervous system is 

 treated under its twofold aspect. 



23. The Cerebro-spinal System is com- 

 posed of the brain and the spinal cord, 

 with the nerves passing from them to 

 the various parts of the body (see Fig. 

 1, p. 8, and Fig. 18). 



24. The Brain fills the cavity of the 

 skull. It consists of five principal 

 parts : (1) the cerebrum, ; (2) the optic 

 thalami, which are so closely united to 

 the cerebrum as to seem to be a part of 

 it ; (3) the optic lobes, or corpora quad- 

 riff emina, and crura cerebri ; (4) the 

 cerebellum, and with it the pans Varolii ; 

 (5) the medulla oblong ata (Figs. 19 and 

 20). Looked at from above or from 

 the side, the only parts of the brain 

 that appear are the cerebrum, a part 

 of the cerebellum, and part of the 

 medulla oblongata. 



25. Cranial Nerves. From the un- 

 der surface of the brain arise twelve 

 pairs of nerves (Fig. 19), which pass 

 through openings in the cranial bones 

 and are distributed in a manner to be 

 described hereafter. They are of three 

 classes : (1) Nerves of special sensa- 

 tion ; (2) motor nerves, that is, nerves 

 which carry nervous impulses to the 

 muscles and cause them to contract; 



Fig. 18. Brain and and (3) mixed nerves, that is, both sen- 

 spinal cord, ventral > 

 canterior) ^ sor V and motor. 



