230 THE CIRCULATION OF THE BLOOD 



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, the maximal 

 speed being at the moment of maximal systolic pressure, see page 228. 



The 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 characterizes the flow in the capillaries and veins. 

 The resistance which is offered to the flow of the blood stream in these 

 vessels is a necessary agent in the production of a continuous stream of 

 blood in the smaller arteries and capillaries. Were there no greater ob- 

 stacle to the escape of blood from the larger arteries than exists to its en- 

 trance 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 

 element in adapting the caliber of the vessels to the quantity of blood which 

 they contain; for the amount of fluid in the blood-vessels varies quite con- 

 siderably 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 

 sometimes inordinately great. The presence of a muscular element, how- 

 ever, provides for a certain uniformity in the amount of pressure exercised: 

 the muscles are in constant action or tone, and it is by this adaptive, uniform, 

 gentle muscular contraction that the normal tone of the blood-vessels is 

 maintained. Deficiency of this tone is the cause of the soft and yielding 

 arterial pulse, and the sluggish blood flow through the arterioles. 



