246 HUMAN PHYSIOLOGY 



less broken up and thus opposes less friction to the inflow 

 of the blood. 



When the veins are reached the propelling force of the 

 heart is still further weakened, and hence the force of inflow 

 is very small. But, instead of there being a resistance to 

 outflow from the veins into the heart, this is favoured by 

 the suction action of the heart during diastole, and also by 

 the fact that the great veins, in entering the heart, pass into 

 the thorax, an air-tight box in which during each inspiration 

 a very low pressure is developed. 



What has been said of the veins applies equally to the 

 lymphatics. 



2. Variations in Blood Pressure. 



Before considering the exact measurements of pressure in 

 these different vessels, the rhythmic variations in pressure 

 may be considered. 



I. Synchronous with the Heart Beats. 



A. Arterial Pulse. 



If the finger be placed on any artery, a distinct expansion 

 will be felt following each ventricular systole. 



This expansion develops suddenly and disappears more 

 slowly. In some cases it may be felt by simply laying the 

 finger on the surface of the artery without exerting pressure, 

 in other cases it may be necessary to compress the artery 

 before the pulsation is distinctly felt. 



If any vein be investigated in the same way it will be 

 found that no pulse can be detected. In the capillaries too 

 this pulse does not exist. 



It is best marked in the great arteries, and becomes less 

 and less distinct as the small terminal arteries are reached. 



Cause of Pulse. The arterial pulse is due to 



1st. The intermittent inflow of blood. The arteries expand 

 with each sudden inflow of 80 grms. of blood from the heart 

 into the arterial system. 



2nd. The resistance to outflow from the arteries into the 

 capillaries. 



If blood could flow freely from the arteries into the capil- 



