THE CIRCULATION 247 



laries, then the inrush of blood from the heart would simply 

 displace the same amount of blood into the capillaries and 

 the arteries would not be expanded. As already indicated, 

 the friction between the walls of the innumerable small 

 arterioles and the blood is so great that the flow out of the 

 arteries is not so free as to allow the blood to pass into the 

 capillaries so rapidly as it is shot into the arteries. Hence 

 with each beat of the heart an excess of blood must 

 accumulate in the arteries. 



3rd. To allow of their expanding to accommodate this 

 excess of blood their walls must be elastic. 



It is upon these three factors that the arterial pulse depends. 

 Do away with either, and the pulse at once disappears. 



Why is there no Pulse in the Veins? Their walls are 

 elastic, but, in the first place, instead of there being an 

 obstruction to the outflow of blood from the veins into the 

 heart, this is favoured by the suction action of the heart and 

 thorax. Hence, even if an intermittent inflow were well 

 marked, the absence of resistance to outflow would in itself 

 prevent the development of a venous pulse. But the inflow 

 is not intermittent. With each beat of the heart the blood 

 does not pass freely from the arteries into the capillaries and 

 veins, but it only slowly escapes, just as much passing out 

 between the beats as during the beats. Hence the most 

 important factor in causing a pulse, an intermittent inflow, 

 is absent. 



With no sudden intermittent inflow, and with no resist- 

 ance to outflow, the development of a pulse is impossible. 



In certain abnormal conditions, where, from the extreme 

 dilatation of the arterioles, the inflow into the veins is very 

 free, and where the outflow from the part of the body is not 

 so free, a local venous pulse may develop. 



Characters of the Pulse Wave. If a finger be placed on 

 the carotid artery and another upon the radial artery it will 

 be felt that the artery near the heart expands (pulses) before 

 that further from the heart. 



The pulse develops first in the arteries near the heart 

 and passes outwards towards the periphery. The reason for 

 this is obvious. The arteries are always overfilled with blood. 

 The ventricle drives its contents into this overfilled aorta, 



