CIRCULATION OF THE BLOOD. 139 



If while the force of the heart remains the same, the rate in- 

 creases, thus increasing the volume of blood in the arteries, the 

 pressure rises. If the rate remains the same, but the volume of 

 blood discharged increases, the pressure will also rise. If the pe- 

 ripheral resistance is increased by contraction of the arterioles the 

 pressure rapidly rises. On the contrary, a diminution in the rate 

 and force of the heart or a diminution in peripheral resistance by a 

 dilatation of the arteries cause a fall in pressure. 



The Pulse. The pulse may be denned as a periodic expansion 

 and recoil of the arterial system. The expansion is caused by the 

 ejection into the arteries of a volume of blood during the systole ; 

 the recoil is due to the reaction of the arterial walls on the blood 

 driving it forward into and through the capillaries, during the 

 diastole. 



At the close of the cardiac diastole the arteries are full of blood 

 and considerably distended. During the occurrence of the succeed- 

 ing systole, the incoming volume of blood is accommodated by a 

 movement forward of a portion of the general blood volume into 

 the capillaries and a further distention of the arteries. The dis- 

 tention naturally begins at the beginning of the aorta. As the blood 

 continues to be discharged from the heart, adjoining segments of 

 the aorta expand in quick succession and by the end of the systole 

 the expansion has travelled over the arterial system as far as the 

 capillaries. This expansion movement which passes over the arterial 

 system in the form of a wave is known as the pulse wave, or the 

 pulse. It is this alternate expansion and recoil which is perceived 

 by the finger when placed over the course of an artery. The artery 

 best adapted for this purpose is the radial as it passes across the 

 wrist joint/ 



The velocity with which the blood flows in the arteries diminishes 

 from the heart to the capillaries, owing to an enlargement in the 

 united sectional area of the vessels ; the velocity increases from the 

 capillaries toward the heart for the opposite reason. The blood moves 

 most rapidly in the large vessels, and especially under the influence of 

 the ventricular systole. From experiments on animals, it has been 

 estimated to move in the carotid of man at the rate of sixteen inches 

 a second, and in the large veins at the rate of four inches a second. 



The caliber of the blood-vessels is regulated by the vasomotor 

 nerves, which have their origin in the gray matter of the medulla 



