THE CIRCULATION OF THE BLOOD. 



119 



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 fr jm the ventricle is, thafc the wall of the first por_ 



FIG. 112. -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 continuous 

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

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

 ed by the dotted lines. 



tion of the artery is dilated into three pouches behind the valves, while 

 the free margins of the valves are drawn inward towards its centre (Fig. 

 112 B). Their positions may be explained by the diagrams, in which 



FIG. 113. View of the base of the ventricular part of the heart, showing the relative position of the 

 arterial and auriculo- ventricular orifices. %. The muscular fibres of the ventricles are exposed by 

 the removal of the pericardium, fat, blood-vessels, etc. ; the pulmonary artery and aorta have been 

 removed by a section made immediately beyond the attachment of the semilunar v'ves, and the 

 auricles have been removed immediately above the auriculo-ventricular orifices. The semilunar 

 and auriculo-ventricular valves are in the nearly closed condition. 1, 1, the base of the right ven- 

 tricle; 1', the conus arteriosus; 2, 2, the base of the left ventricle; 3, 3, the divided wall of the right 

 auricle; 4, thatof the left; 5, 5'. 5", the tricuspid valve; 6, 6', the mitral valve. In the angles be- 

 tween these segments are seen the smaller fringes frequently observed; 7, the anterior part of the 

 pulmonary artery; 8, placed upon the posterior part or the root of the aorta; 9, the right, 9', the left 

 coronary artery. (Allen Thomson.) 



the continuous lines represent a transverse section of the arterial walls, 

 the dotted ones the edges of the valves, firstly, when the valves are near- 

 est to the walls (A), as in the dead heart, and, secondly, when, the walls 

 being dilated, the valves are drawn away from them (B). 



