Il6 THE CIRCULATION. 



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 unyield- 

 ing 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 



Fig. 37.* 



centre (fig. 37, B). Their positions may be explained by 

 the foregoing diagrams, 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 in contact with the walls (A), and, secondly, 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, so- 



* Fig. 37. Sections of aorta, to show the action of the semilunar 

 valves. A is intended to show the valves, represented by the dotted 

 line-, 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 equilateral triangle, as represented by the dotted 

 lines. 



