262 VETEEINAEY PHYSIOLOGY 



If the finger be placed on an artery, a distinct expansion will 

 be felt following each systole, and due to this rise of pressure. 

 This is the arterial pulse. 



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 marked 

 pressure, in other cases it may be necessary to compress the 

 artery before the pulsation is distinctly felt. 



If a vein be investigated in the same way it will be found 

 that no such 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 

 from the sudden increase of pressure due to each sudden flow 

 of 80 grins, of blood from the heart into the arterial system. 



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

 capillaries. 



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

 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 any of them, 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 



