THE CIRCULATION 99 



organs whose blood tubes, not being enlarged just then, 

 afford greater resistance to the passage of the blood than 

 the dilated vessels afford. But can the total amount of 

 blood reaching, in a given time, not a single organ only, 

 but all the organs in the body, be increased ? Yes, for 

 if the one and one fourth gallons of blood flow faster all 

 over the body, more fresh blood will reach each organ. 

 This is accomplished by faster or stronger beating of 

 the heart. During rapid general exercise, as running, 

 when the demands of the body are increased, the heart 

 beats faster, as you have doubtless observed. When the 

 body in general is at rest, as during sleep, the heart 

 beats more slowly. Thus the general blood supply is 

 regulated. But how is the heart itself regulated ? For 

 these facts show two things : first, that we cannot directly 

 control it by the will; and second, that there is some- 

 thing in the body that does control the heart, and perhaps 

 our wills may influence the beating of the heart indirectly. 

 The heart, like the blood vessels and the muscles in gen- 

 eral, is supplied with nerves ; but there is this difference, 

 namely, that the heart can go on beating without receiv- 

 ing impulses along its nerves. The heart of a frog, after 

 being cut out of the body, will go on beating for several 

 hours if it is kept moist; if it is cut into several pieces, 

 the pieces will go on beating. It is the property of the 

 heart muscle to contract, and it will do so as long as its 

 protoplasm is alive. 



163. The nerves going to the heart do indeed carry 

 impulses to the heart, but this is to regulate the beat of the 

 heart and not to originate it. A nerve called the vagus 

 nerve, extending from the spinal bulb to several organs, 

 goes to the heart ; and gentle impulses which are almost 

 always passing down the vagus nerve from the spinal bulb, 

 restrain the heart from too great activity, and are the 

 chief means of regulating the strength and frequency of 



