THE CIRCULATION OF THE BLOOD. 117 



tity of blood remains. The shape of both ventricles during systole 

 undergoes an alteration, the left probably not altering in length but to a 

 certain degree in breadth, the diameters in the plane of the base being 

 diminished. The right ventricle does actually shorten to a small extent. 

 The systole has the effect of diminishing the diameter of the base, espe- 

 cially in the plane of the auriculo-ventricular valves; but the length of 

 the heart as a whole is not altered. (Ludwig. ) During the systole of 

 the ventricles, too, the aorta and pulmonary artery, being filled with 

 blood by the force of the ventricular action against considerable resist- 

 ance, elongate as well as expand, and the whole heart moves slightly to- 

 wards the right and forwards, twisting on its long axis, and exposing 

 more of the left ventricle anteriorly than is usually in front. When the 

 systole ends the heart resumes its former position, rotating to the left 

 again as the aorta and pulmonary artery contract. 



Functions of the Valves of the Heart. (1) The Auricula- Ventric- 

 ular. The distention of the ventricles with blood continues throughout 

 the whole period of their diastole. The auriculo-ventricular valves are 

 gradually brought into place by some of the blood getting behind the 

 cusps and floating them up; and by the time that the diastole is complete, 

 the valves are no doubt in apposition, the completion of this being 

 brought about by the reflux current caused by the systole of the auricles. 

 This elevation of the auriculo-ventricular valves is 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 recollected that the diminution in the breadth of the base 

 of the heart in its transverse diameters during ventricular systole is es- 

 pecially marked in the neighborhood of the auriculo-ventricular rings, 

 and this aids in rendering 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 an- 

 other, by the simultaneous contraction of the musculi papillares, whose 

 chordae tendineaB have a special mode of attachment for this object (p. 

 104). The cusps of the auriculo-ventricular valves meet not by their 

 edges only, but by the opposed surfaces of their thin outer borders. 



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

 of the ventricle no doubt help to produce the obliteration of the ventric- 

 ular cavity during 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 rigor mortis is very marked (Fig. 91). 

 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 



