102 THE CIRCULATION. 



The arterial or semilunar valves are, as already said, brought 

 into action by the pressure of the arterial blood forced back 

 towards the ventricles, when the elastic walls of the arteries 

 recoil after being dilated by the blood propelled into them in 

 the previous contraction of the ventricle. The dilatation of 

 the arteries is, in a peculiar manner, adapted to bring the 

 valves into action. The lower borders of the semilunar valves 

 are attached to the inner surface of a tendinous ring, which is, 

 as it were, inlaid, at the orifice of the artery, between the mus- 

 cular fibres of the ventricle and the elastic fibres of the walls 

 of the artery. The tissue of this ring is tough, does not admit 

 of extension under such pressure as it is commonly exposed to ; 

 the valves are equally inexteusile, being, as already mentioned, 

 formed of tough, close-textured, fibrous tissue, with strong in- 

 terwoven cords, and covered with endocardium. Hence, when 

 the ventricle propels blood through the orifice and into the 

 canal of the artery, the lateral pressure which it exercises is 

 sufficient to dilate the walls of the artery, but not enough to 

 stretch in an equal degree, if at all, the unyielding valves and 

 the ring to which their lower borders are attached. The effect, 

 therefore, of each such propulsion of blood from the ventricle 

 is, that the wall of the first portion of the artery is dilated into 

 three pouches behind the valves, while the free margins of the 

 valves, which had previously lain in contact with the inner 

 surface of the artery (as at A, Fig. 37), are drawn inward 

 towards its centre (Fig. 37, B). Their positions may be ex- 



FlG. 37. 



Sections of aorta, to show the action of the semilunar valves. A is intended to 

 show the valves, represented by the dotted lines, in contact with the arterial walls, 

 represented by the continuous outer line. B (after Hunter) shows the arterial wall 

 distended into three pouches (a), and drawn away from the valves, which are straight- 

 ened into the form of an equilateral triangle, as represented by the dotted lines. 



plained by the foregoing diagrams, in which the continuous 

 lines represent a transverse section of the arterial walls, the 



