THE HEART AND THE BLOOD VESSELS 145 



nervous system. The nerves from the brain, the vagi (Fig. 77), 

 act as a brake on the heart; if they are artificially stimulated, 

 the beat of the heart is retarded; or if they are severely 

 irritated, heart action may stop altogether for a short time, 

 although after a little the heart escapes from its influence and 

 goes on beating again. The office, then, of the vagus nerves 

 is to keep the heart from beating too rapidly. In a healthy 

 body they would never stop the action as described above, 

 but the slowing influence is necessary when the body tissues 

 need but little blood, as when one is idle or asleep. The 

 vagus nerves, because they check the heart action, are called 

 the inhibitor nerves. 



The second pair of nerves which affect the heart are called 

 the sympathetic nerves; Fig. 77. If these nerves are stimu- 

 lated, they make the heart beat faster and more forcibly, for 

 which reason they are sometimes called the accelerator nerves. 



Thus we find that three influences are constantly affecting 

 and regulating the heart: 



1. The impulse to beat, located within the heart itself. 



2. The inhibitory influences, which reach it from the cen- 

 tral nervous system over the vagus nerves. 



3. The accelerator influences, which come over fibres by 

 way of the sympathetic nerves. 



All these actions take place unconsciously, for one has no 

 power voluntarily to modify the activity of the heart. If 

 the heart is thus made to beat more or less rapidly, it, of 

 course, affects the rate of circulation. The more rapidly the 

 heart beats, the more rapidly the blood circulates, and a 

 slowing of the heart beat will check the circulation of the blood. 



THE BLOOD VESSELS 



Ah 1 vessels in the body which conduct blood, "pure" or 



"impure," away from the heart are called arteries; all which 



,! carry blood to or toward the heart are called veins. Both 



are large in the region of the heart; but if one follows the ar- 



