THE NERVOUS SYSTEM 



123 



bellum. The cerebrum covers the cerebellum. (Fig. 

 1 10.) Is this true of the monkey's brain? (See Fig. 



The work of the medulla is chiefly to control the vital 

 functions (see Figs, no, 114). Here are located the 

 centers for regulating the 

 breathing, the heart beat, the 

 size of the blood vessels (thus 

 regulating nutrition), and also 

 the less important centers 

 that control swallowing, secre- 

 tion of saliva, and vomiting. 

 The center for breathing is 

 sometimes called the vital 

 knot, because although the 

 cerebrum and cerebellum may be removed from an animal 

 without causing immediate death, the slightest injury 

 to the vital knot kills the animal at once. In cases of 



hanging, death is caused by 

 injury to this center. 



Automatic Action. The 

 center called the vital knot 

 is said to regulate the 

 breathing automatically, not 



FIG. no. THE BRAIN (cerebrum, 

 cerebellum, medulla). 



FIG. in. ASSOCIATION FIBERS, con- 

 necting cells within the cerebrum. 

 (Jegi.) 



reflexly. Reflex acts start 

 in the skin ; automatic acts 

 start in the interior of the 

 body. The condition of the blood regulates the breathing 

 automatically during sleep, and partly regulates it during 

 waking. If too much carbon dioxid accumulates in the 

 blood this excites the vital knot, which sends out stronger 

 impulses to the respiratory muscles. Deeper breathing 

 follows, which purifies the blood, and the breathing is then 



