CIRCULATION OF THE BLOOD. 113 



riculo-ventricnlar valves is, no doubt, materially aided by the action of the 

 elastic tissue which has been shown to exist so largely in their structure, 

 especially on the auricular surface. At any rate at the commencement 

 of the ventricular systole they are completely closed. It should be recol- 

 lected that the diminution in the breadth of the base of the heart in its 

 transverse diameters during ventricular systole is especially marked in the 

 neighborhood of the auriculo- ventricular rings, and thus aids in render- 

 ing the auriculo-ventricular valves competent to close the openings, by 

 greatly diminishing their diameter. The margins of the cusps of the 

 valves are still more secured in apposition with another, by the simulta- 

 neous contraction of the musculi papillares, whose chordae tendineae have a- 

 special mode of attachment for this object (p. 110). As in the case of 

 the semilunar valves to be immediately described, the auriculo-ventricular 

 valves meet not by their edges only, but by the opposed surfaces of their 

 thin outer borders. The semilunar valves, on the other hand, which are 

 closed in the intervals of the ventricle's contraction (Fig. 92, 6), are 

 forced apart by the same pressure that tightens the auriculo-ventricular 

 valves; and, thus, the whole force of the contracting ventricles is directed 

 to the expulsion of blood through the aorta and pulmonary artery. 



The form and position of the fleshy columns on the internal walls of 

 the ventricle no doubt help to produce this obliteration of the cavity dur- 

 ing their contraction; and the completeness of the closure may often be 

 observed on making a transverse section of a heart shortly after death, in 

 any case in which the contraction of the rigor mortis is very marked (Fig. 

 94). In such a case only a central fissure may be discernible to the eye 

 in the place of the cavity of each ventricle. 



If there were only circular fibres forming the ventricular wall, it is 

 evident that on systole the ventricle would elongate; if there were 

 only longitudinal fibres the ventricle would shorten on systole; but there 

 are both. The tendency to alter in length is thus counterbalanced, and 

 the whole force of the contraction is expended in diminishing the cavity 

 of the ventricle; or, in other words, in expelling its contents. 



On the conclusion of the systole the ventricular walls tend to expand 

 by virtue of their elasticity, and a negative pressure is set up, which tends 

 to suck in the blood. This negative or suctional pressure on the left side 

 of the heart is of the highest importance in helping the pulmonary cir- 

 culation. It has been found to be equal to 23 mm. of mercury, and is 

 quite independent of the aspiration or suction power of the thorax in aid- 

 ing the blood-flow to the heart, to be described in the chapter on Respira- 

 tion. 



Function of the Musculi Papillares. The special function of 



the musculi papillares is to prevent the auriculo-ventricular valves from 



being everted into the auricle. For the chordae tendineae might allow 



the valves to be pressed back into the auricle, were it not that when the 



VOL. I. 8. 



