112 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



of the lunula being one-half of the free margin of the segment (see Fig. 18). 

 Over the surface of each lunula each segment is in contact with a different 

 one of ils two fellows (see Fig. 20). The firmness of closure thus secured is 

 shown by Figure li», which represents a longitudinal section of the artery, 

 passing through two of the closed segments. The forces which press together 

 the opposed surfaces are equal and opposite, and the parts of the segments 

 which correspond to these surfaces undergo do strain. The Lunula?, therefore, 

 like the mutually opposed portions of the mitral or tricuspid valve, are very 

 delicate and flexible, while the rest of each semilunar segment is strongly 

 made, to resist of itself the arterial pressure. 



Corpora Arantii and their Uses. — At the centre of the free margin of 

 each semilunar segment, just between the ends of the two lunula?, there is a 

 small thickening, more pronounced in the aorta than in the pulmonary artery, 

 called the " body of Aranzi " * (corpus Arantii). This thickening both rises 

 above the edge and projects from the surface between the lunula?. When the 

 valve is closed, the three corpora Arantii come together and exactly fill a small 

 triangular chink, which otherwise might be left open just in the centre of the 

 cross section of the artery (see Figs. 18, 20). 



The foregoing shows that the mechanism of the semilunar valves is no less 

 effective, though far simpler, than that of the mitral and tricuspid. That the 

 latter two should be more complex is natural ; for each of them must give 

 free entrance to and prevent regurgitation from a chamber which nearly 

 empties itself, and hence undergoes a very great relative change of volume ; 

 while the arterial system is at all times distended and undergoes a change of 

 capacity which is relatively small while receiving a pulse-volume and trans- 

 mitting it to the capillaries. 



I. The Changes in Form and Position of the Beating Heart, and 



the Cardiac Impulse. 



General Changes in the Heart and Arteries. — During the brief systole 

 of the auricles these diminish in size while the swelling of the ventricles is 

 completed. During the more protracted systole of the ventricles, which imme- 

 diately follows, these diminish in size while the auricles are swelling and the 

 injected arteries expand and lengthen. During the greater part of the suc- 

 ceeding diastole of the ventricles both these and the auricles are swelling, and 

 all the muscular fibres of the heart are flaccid, up to the moment when a new 

 auricular systole completes the diastolic distention of the ventricles, as above 

 stated. During the ventricular diastole, as the great arteries recoil they 

 shrink and shorten. The changes of size in the beating heart depend entirely 

 upon the changes in the volume of blood contained in it, and not upon changes 

 in the volume of the muscular walls. The muscular fibres of the heart agree 

 with those found elsewhere in not changing their volume appreciably during 

 contraction, but their form only. The cardiac cycle thus runs its course with 



1 Named from Julius Ca?sar Aranzi of Bologna, an Italian physician and anatomist, bom 

 in 1530. 



