THE BRAIN 2QI 



522. The vagus nerve. One of the pairs of cranial nerves is 

 called the vagus, or pneumogastric, nerve. It supplies a small sensory 

 branch to the ear, and motor branches to the larynx and pharynx ; 

 then it passes into the thorax and gives off branches to the heart, 

 which restrain its action. It gives sensory branches to the esophagus 

 and lungs, and finally reaches the stomach and liver. The main nerve 

 supply of these organs is from the spinal cord, or from the sympathetic 

 system, but the vagus nerve is an additional means for better regulating 

 their action to suit the needs of the body. 



523. Centers originating impulses. In the medulla, a 

 collection of nerve cells, called the respiratory center, 

 sends out a regular succession of orders for respiratory 

 movements. While the orders may be hastened or re- 

 tarded by other nerve centers to suit the needs of the 

 body, yet the medulla compels the respiratory muscles to 

 act so as to keep the body supplied with sufficient oxy- 

 gen. Thus it is a real part of the brain. When the 

 respiratory center is destroyed, respiration and life cease 

 instantly. 



There is another part of the medulla, called the vaso- 

 motor center, which controls the contraction of arteries, 

 and another which regulates the peristalsis of the esoph- 

 agus in swallowing. While these are partly reflex acts, 

 yet their perfect action requires original impulses to be 

 sent from the medulla. 



524. Effects of reflex influences. The respiration, circula- 

 tion of the blood, and taking of food are essential vital processes of 

 life which the medulla controls without our being aware of it. Strong 

 influences from the nerves of the body may act in a reflex way to 

 modify the impulses of the medulla. Great fear may cause the vaso- 

 motor center to send out impulses for the contraction of the arteries so 

 as to produce great paleness. Instances have occurred in which the 

 disturbance of circulation from this cause has produced death. 



525. Effects of injury. An injury to the respiratory and vaso- 

 motor centers causes death at once. A broken neck, if high up, may 



