192 HANDBOOK OF PHYSIOLOGY. 



The lower borders of the semilunar valves are attached to the inner 

 surface of the tendinous ring, which is, as it were, inlaid at the orifice 

 of the artery, between the muscular fibres of the ventricle and the 

 elastic fibres of the walls of the artery. The tissue of this ring is tough, 

 and does not admit of extension under such pressure as it is commonly 

 exposed to; the valves are equally inextensile, being, as already men- 

 tioned, formed mainly of tough, close-textured, fibrous tissue, with 

 strong interwoven cords. 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, there- 

 fore, 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 



Fig. 164. Sections of aorta, to show the action of the semilunar valves. A is intended to show 

 the valves, represented by the dotted lines, lying near the arterial walls, represented by the contin- 

 uous outer line. B (after Hunter) shows the arterial wall distended into three pouches (a), and 

 drawn away from the valves, which are straightened into the form of an equilateral triangle as 

 represented by the dotted lines. 



the valves, while the free margins of the valves are drawn inward toward 

 its centre (fig. 164, B). Their positions may be explained by the dia- 

 grams, in which the continuous lines represent a transverse section of 

 the arterial walls, the dotted ones the edges of the valves, firstly, when 

 the valves are nearest to the walls (A), as in the dead heart, and, sec- 

 ondly, when, the walls being dilated, the valves are drawn away from 

 them (B). 



This position of the valves and arterial walls is retained so long as 

 the ventricle continues in contraction : but as soon as it relaxes, and the 

 dilated arterial walls can recoil by their elasticity, the blood is forced 

 backward toward the ventricles and onward in the course of the circu- 

 lation. Part of the blood thus forced back lies in the pouches (sinuses 

 of Valsalva) (, fig. 164, B) between the valves and the arterial walls; 

 and the valves are by it pressed together till their thin lunated margins 

 meet in three lines radiating from the centre to the circumference of 

 the artery (7 and 8, fig. 148). 



The contact of the valves in this position and the complete closure 



