CIRCULATION OF BLOOD AND OF LYMPH 



157 



The Pulse. In an earlier section the real cause of the pulse, 

 i.e. the constantly alternating increase and decrease in size 

 of the arteries due to heart beat, has been 

 noted. There is, however, no pulse in the 

 capillaries. Such a constant 

 stretching and swelling 

 would break down their thin 

 walls. In the veins, too, 

 since they are formed by the 

 running together of the capil- 

 laries, no pulse is present. 

 Two agents have brought 

 about this disappearance of 

 pulse in the veins, (1) the 

 elasticity of the arterial 

 walls, and (2) the opposi- 

 tion which the small arter- 

 ies arterioles offer to on- 

 ward blood flow. If the 

 arteries had rigid walls, there 

 would be pulse in the capil- 

 laries and veins, in spite of 

 the small calibre of the arte- 

 rioles; and if the capillaries 

 and small arteries were large, 

 the pulse would be car- 

 ried over into the veins in 

 spite of the elasticity of the arterial walls. 

 We may note here also that the pulse is not 

 a simple throb, but consists of two parts; first, 

 strong, abrupt "beat," and immediately following, a second 

 sveaker beat." These cannot be felt as separate with the 

 naked finger, but are detected with a pulse recording apparatus. 

 The first part of the pulse wave is caused by the sudden rush of 

 blood into the arteries when the ventricle contracts, while the 



FIG 



, 



87. THE 

 ARM FROM IN 



FRONT 



Showing the chief 

 arteries. (Modi- 

 fied from Tiede- 

 mann) 



FIG. 88. THE 

 THIGH AND 

 KNEE FROM 

 THE INSIDE 



Showing arteries. 

 (Modified from 

 Tiedemann) 



