74 



Physiology. 



currents that it receives through the sympathetic nerves. 

 When we exercise actively, the fact that the muscles need 

 more blood is telegraphed both to 

 the heart and to the arteries leading 

 to the muscles of the arteries, and 

 they are regulated accordingly. 



How the Heart is made to Beat 

 Slower. The slowing of the beat of 

 the heart is due to other nerves, not 

 belonging to the sympathetic system. 

 The vagus nerves are a pair 

 of cranial nerves. They arise 

 from the sides of the spinal 

 bulb, at the base of the brain, 

 and, passing downward, give 

 branches to the gullet, stomach, 

 lungs, and . heart. The distri- 

 bution of the vagus nerves is 

 shown in Fig. 47. Nerve cur- 

 rents reaching the heart through the 

 vagus nerves make it beat slower, 

 and if the current is strong enough, 

 as in case of a severe blow over the 

 stomach, may, by reflex action, stop Flgf- 47 ' 

 the heart. 



Lungs 



Heart 



Liver 



Stomach 



of vagus 



Influence of Gravity on Circulation. Although the heart 

 pumps the blood around through the body independent of 

 the force of gravity, yet the circulation is influenced by this 

 force. For instance, a person who has fainted should ,be 

 laid flat on his back, that the heart may more easily drive 

 blood to the brain. A sore hand feels less pain if held up, 

 as in a sling, than when hanging by the side, and a sprained 



