ELASTICITY OF ARTERIES. 117 



of the force impelling the blood, and exhaust it before they 

 are in danger of bursting, through being overstretched. Elas- 

 ticity is thus advantageous in all arteries, but chiefly so in the 

 aorta and its large branches, which are provided, as already 

 said, with a large proportional quantity of elastic tissue, in 

 adaptation to the great force of the left ventricle, which falls 

 first on them, and to the increased pressure of the arterial 

 blood in violent expiratory efforts. 



On the subsidence of the pressure, when the ventricles cease 

 contracting, the arteries are able, by the same elasticity, to 

 resume their former calibre; and in thus doing, they manifest, 

 2d, the chief purpose of their elasticity, that, namely, of equal- 

 izing the current of the blood by maintaining pressure on the 

 blood in the arteries during the periods at which the ventricles 

 are at rest or dilating. If some such method as this had not 

 been adopted if, for example, the arteries had been rigid 

 tubes, the blood, instead of flowing as it does, in a constant 

 stream, would have been propelled through the arterial system 

 in a series of jerks corresponding to the ventricular contrac- 

 tions, with intervals of almost complete rest during the inaction 

 of the ventricles. But in the actual condition of the arteries, 

 the force of the successive 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 contractions of the ventricles, the force of the re- 

 coiling and contracting arteries is employed in continuing the 

 same direct propulsion. Of course, the pressure exercised by 

 the recoiling arteries is equally diffused in every direction 

 through the blood, and the blood would tend to move back- 

 wards as well as onwards, but that all movement backwards 

 is prevented by the closure of the semilunar arterial valves, 

 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, according to that law of action of 

 elastic bodies, by which they return to the state of rest with a 

 force equal to that by which they were disturbed therefrom. 

 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 ventricles; they would not contract 

 had they not first been dilated, any more than a spiral spring 

 would shorten itself unless it were first elongated. The advan- 

 tage of elasticity in this respect is, therefore, not that it in- 



