CIRCULATION OF THE BLOOD. 125 



The Pulse. The pulse may be defined 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 con- 

 siderably distended. During the occurrence of the succeeding 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 disten- 

 tion of the arteries. The distention 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 sys- 

 tole the expansion has travelled over the arterial system as far as the capil- 

 laries. 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 alter- 

 nate 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 peripheral arteries is regulated by the vasomotor 

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

 They issue from the spinal cord through the anterior roots of spinal nerves 

 from the second thoracic to the third or fourth lumbar nerve, pass through 

 the sympathetic ganglia in different situations around the nerve cells of 

 which they arborize. From the ganglion cells new fibers arise which ulti- 

 mately are distributed to the coats of the blood-vessels. They exert at 

 different times a constricting and a dilating action upon the vessels, thus 

 keeping up the arterial tonus and the average blood pressure. 



Capillaries. The capillaries constitute a network of vessels of micro- 

 scopic size, which distribute the blood to the inmost recesses of the tissues, 

 inosculating with the arteries on the one hand and the veins on the other; 

 they branch and communicate in every possible direction. 



