198 THE CIRCULATION OF 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 

 cannot 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 187. 



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 resist- 

 ance 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 cooperates with the elastic 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 considerably even 

 from hour to hour, and can never be quite constant; and 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- 

 times inordinately great. The presence of a muscular element, however, 

 provides for a certain uniformity in the amount of pressure exercised; 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. 



Incidentally it may be mentioned that the elastic and muscular contrac- 

 tion of an artery may also be regarded as fulfilling a natural purpose when, 

 the artery being cut, the sudden contraction at first limits, and then, in con- 

 junction with the coagulated fibrin, completely arrests, the flow of blood. 

 It is only in consequence of such contraction and coagulation that we are 



