THE ARTERIAL FLOW IS RHYTHMIC 2 19 



cles are at rest or are dilating. If the arteries were rigid tubes, the blood, 

 instead of flowing as it does in a constant stream, would be propelled through 

 the arterial system in a series of spurts corresponding in time to the ventric- 

 ular contractions and with intervals of almost complete rest during the in- 

 action of the ventricles. But in the actual condition of the vessels, the force 

 of the successive contractions of the ventricles is expended partly in the 

 direct propulsion of the blood and partly in the dilatation of the elastic ar- 

 teries; and in the intervals between the contractions of the ventricles, the 

 force of the recoil is employed in continuing the flow onward. Of course 

 the pressure exercised is equally diffused in every direction, and the blood 

 tends to move backward as well as onward. All movement backward, 

 however, is prevented by the closure of the semilunar valves, which takes 

 place at the very commencement of the recoil of the arterial walls. 



The Arterial Flow is Rhythmic. By the exercise of the elasticity 

 of the arteries, all the force of the ventricles is expended upon the circulation. 

 That part of the force which is used up or rendered potential in dilating the 

 arteries is restored or made active or kinetic when they recoil. There is no 

 loss of force, neither is there any gain; for the elastic walls of the artery can- 

 not originate any force for the propulsion of the blood; they only restore 

 that which they receive from the ventricles. 



Since the ventricular discharge is intermittent, there will be intermittent 

 accessions of pressure, and therefore the flow of blood in the arteries will 

 be periodically accelerated. The volume of blood discharged from a cut 

 artery increases and decreases with the systole and diastole of the ventricles, 

 or with the systolic and diastolic pressures of the arteries themselves, see 

 page 215. 



This equalizing influence of the resistance of the successive arterial 

 branches reacts so that at length the intermittent accelerations produced in 

 the arterial flow by the discharge of the heart cease to be observable, and 

 the jetting stream is converted into the continuous and even movement of 

 the blood which we see in the capillaries and veins. In the production of a 

 continuous stream of blood in the smaller arteries and capillaries, the re- 

 sistance which is offered to the blood stream in these vessels is a necessary 

 agent. Were there no greater obstacle to the escape of blood from the 

 larger arteries than exists to its entrance into them from the heart, the 

 stream would be intermittent, notwithstanding the elasticity of the walls of 

 the arteries. 



The muscular element of the middle coat co-operates with the elastic in 

 adapting the caliber of the vessels to the quantity of blood which they con- 

 tain; for the amount of fluid in the blood vessels varies quite considerably 

 even from hour to hour, and can never be quite constant. Were the elastic 

 tissue only present, the pressure exercised by the walls of the containing 

 vessels on the contained blood would be sometimes very small and some- 



