54 



ELEMENTARY PHYSIOLOGY 



to fl,ap back and so destroy their utility. This tendency, 

 however, is counteracted by the rhurdce tendincce being 

 connected, not directly to the walls of the heart, but to 

 those muscular pillars, the papillary mnsdcs, which stand 

 out from its substance. These muscular pillars shorten 

 at the same time as the substance of the heart contracts ; 

 and thus, just so far as the contraction of the walls of the 

 ventricles brings the papillary muscles nearer the valves, 

 do they, by their own contraction, pull the choroUe ten- 

 diiiece as tight as before. 



m- 



FiG. 21.— Diagram to ilhstrate the Action of the Heart. 



aur. .■wiride ; rent, ventricle; VV. veins; a, aorta; m, mitral valve; 

 », semilunar valve. 



In A the aviricle is contracting, ventricle dilated, mitr.al valve open, 

 semilunar valves closed. In B the auricle is dilated, ventricle contracting, 

 mitral valve elosed, semilunar valves open. 



By the means which have now been described, the fluid 

 in the ventricle is debarred from passing back into the 

 auricle ; the whole foi-ce of the contraction of the ventri- 

 cular walls therefore is exjiended in overcoming the resist- 

 ance presented by the semilunar valves. This resistance 

 is partly the result of the mere weight of the vei-tical 

 column of blood which the valves support ; but is chiefly 

 due to the reaction of the distended elastic walls of the 



