BLOOD PRESSURE. 295 



city throughout, it would be impossible for the heart to empty the 

 veins, to overfill the arteries, and to establish the great pressure- 

 difference that normally exists. Therefore some resistance equal 

 to the pressure must be offered to the flow of the blood from the 

 arteries into the veins. 



This resistance is made up of several items, of which one alone 

 is sufficient to keep up the arterial pressure, namely, the active 

 contraction of the arterioles. No doubt the enormous increase of 

 surface over which the blood has to move in the capillaries, and 

 the pressure exercised upon them by the surrounding elastic tis- 

 sues, impede the emptying of the arteries. But it will be seen 



Fir.. 1^0 



Tracing showing the effect of Stimulation of Vagus Nerve. Stimulus applied between 

 vertical linea. (Recording surface moved from left to right.) 



from the following consideration that the contractility of the 

 arterioles is the most important item. The resistance offered by 

 the capillaries is insignificant when compared with the high arterial 

 blood pressure, for the increase of friction accompanying their 

 greater extent of surface is counterbalanced by the decrease of 

 friction dependent upon the much greater calibre of the capil- 

 laries, compared with that of the arterioles which have to bear 

 the brunt of the force of the arterial current, and, further, the 

 capillary friction is far from sufficient to restrain the blood from 

 rushing into the veins. This is seen when the arterioles are par- 

 alyzed by the destruction of the nervous mechanism controlling 



