CIRCULATION OF THE BLOOD. 



139 



pelled is directed against the sides of the arteries; under this force their 

 elastic walls dilate, stretching enough to receive the blood, and as they 

 stretch, becoming more tense and more resisting. Thus, by yielding, 

 they break the shock of the force impelling 

 the blood. On the subsidence of the pressure, 

 when the ventricles cease contracting, the arte- 

 ries aiv able, by the same elasticity, to resume 

 their former calibre; (b) It equalizes the cur- 

 rent of the blood by maintaining pressure on 

 it in the arteries during the periods at which 

 the ventricles are at rest or dilating. If the 

 arteries had -been rigid tubes, the blood, in- 

 stead of flowing, as it does,, in a constant 

 stream, would have been propelled through 

 the arterial system in a series of jerks corre- 

 sponding to the ventricular contractions, w r ith 

 intervals of almost complete rest during the 

 inaction of the ventricles. But in the actual 

 condition of the arteries, the force of the suc- 

 cessive contractions of the ventricles is ex- 

 pended partly in the direct propulsion of the 

 blood, and partly in the dilatation of the elastic 

 arteries; and in the intervals between the con- 

 tractions of the ventricles, the force of the re- 

 coil is employed in continuing the same direct 

 propulsion. Of course, the pressure they ex- 

 ercise is equally diffused in every direction, 

 and the blood tends to move backward as well 

 as onward, but all movement backward is pre- 

 vented by the closure of the semilunar arterial valves (p. 114), which 

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

 By this exercise of the elasticity of the arteries, all the force of the 

 ventricles is made advantageous to the circulation; for that part of their 

 force which is expended in dilating the arteries, is restored in full when 

 they recoil. There is thus no loss of force; but neither is there any gain, 

 for the elastic walls of the artery cannot originate any force for the propul- 

 sion of the blood they only restore that which they received from the 

 ventricles. The force with which the arteries are dilated every time the 

 ventricles contract, might be said to be received by them in store, to be all 

 given out again in the next succeeding period of dilatation of the ventricles. 

 It is by this equalizing influence of the successive branches of every artery 

 that, at length, the intermittent accelerations produced in the arterial 

 current by the action of the heart, cease to be observable, and the jetting 

 stream is converted into the continuous and equable movement of the 



Fi. 121. Surface view of an 

 artery from the mesentery of a 

 frog, ensheathed in a perivascular 

 lymphatic vessel, a. The artery, 

 with its circular muscular coat 

 (media) indicated by broad trans- 

 verse markings, with an indication 

 of the adventitia outside. I. Lym- 

 phatic vessel ; its wall is a simple 

 endothelial membrane. (Klein and 

 Noble Smith.) 



