THE CIRCULATION OF THE BLOOD. 135 



of the blood, and partly in the dilatation of the elastic arteries ; and in 

 the intervals between the contractions of the ventricles, the force of the 

 recoil is employed in continuing the same direct propulsion. Of course 

 the pressure they exercise is equally diffused in every direction, and the 

 blood tends to move backwards as well as onwards, but all movement 

 backwards is prevented by the closure of the aortic semi-lunar valves 

 (p. 104), 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 expended upon the circulation ; for that part of their force 

 which is used 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 propulsion 

 of the blood they only restore that which they received from the ven- 

 tricles. 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 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 resistance which is offered to the blood-stream in these 

 vessels, is a necessary agent. Were there no greater obstacle to the es- 

 cape of blood from the larger arteries than exists to its entrance into 

 them from the heart, the stream would be intermittent, notwithstand- 

 ing the elasticity of walls of the arteries. 



(c.) By means of the elastic and muscular tissue in their walls the 

 arteries are enabled to dilate and contract readily in correspondence 

 with any temporary increase or diminution of the total quantity of blood 

 in the body ; and within a certain range of diminution of the quantity, 

 still to exercise due pressure on their contents; (d.) The elastic tissue 

 assists in restoring the normal state after diminution of its calibre, 

 whether this has been caused by a contraction of the muscular coat, or 

 the temporary application of a compressing force from without. This 

 action is well shown in arteries which, having contracted by means of 

 their muscular element, after death, regain their average patency on the 

 cessation of post-mortem rigidity, (e.) By means of their elastic coat 

 the arteries are enabled to adapt themselves to the different movements 

 of the several parts of the body. 



The natural state of all arteries, in regard at least to their length, is 

 one of tension they are always more or less stretched, and ever ready 

 to recoil by virtue of their elasticity, whenever the opposing force is re- 



