THE HEART AND CIRCULATION. 113 



less constant. The insufficient outlet also helps to make 

 the flow constant. 



By the time the blood reaches the veins its rhythmic 

 character has been done away with, but though there are 

 no elastic walls in the veins, it still has force enough after 

 the slowing in the capillaries to return to the heart. 

 In this it is aided to a certain extent by the valves and 

 by the action of the skeletal muscles as they contract and 

 expand, especially in the arms and legs, where the blood 

 runs perpendicularly and there is a high column to be 

 supported There are also more veins than arteries, each 

 large artery having two large veins, the venae comites, to 

 help get the blood back to the heart, and the veins anas- 

 tomose freely. Thus, if the blood cannot get back by one 

 channel it does by another. In parts like the brain, where 

 it is very important that there should be no compression, 

 since any disturbance of circulation would lead to serious 

 results, the vessels are enclosed in thick walls, and in the 

 liver, through which all the blood passes and where com- 

 pression is sure to cause trouble, the veins are simply 

 caverns carved out in the organ and have no walls. 

 They lie open when the organ is opened. Varicose 

 veins are the result of valves giving way through inherited 

 weakness or disease so that others have an unduly large 

 weight to support. 



The Pulse. The pulse wave is characterized by a 

 quick rise and a slow fall, though this cannot ordinarily 

 be distinguished by the finger. In some slow fevers, how- 

 ever, the fall is very long and distinct ripples can be felt. 

 This is known as the dicrotic pulse. With age the 

 arterial walls grow stiffer and more rigid and less adapted 

 to their work. In certain cases of heart disease the heart 

 does not transmit all the beats to the pulse and to get the 

 true rate the heart must be listened to. 



The rate at which the pulse wave travels varies with 

 the size of the artery and the force of the heart beat 

 but is about 15 to 20 feet a second. The flow is most 

 rapid in the arteries because they are nearest the heart, 



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