272 



THE MECHANICS OF THE HEART 



eral segment. The latter peculiarity is dependent upon the fact 

 that the wall opposite each valvular segment is distended to form a 

 pKDcket, the so-called sinus of Valsalva.^ A certain quantity of residual 

 blood is always retained in these enlargements. From the right and 

 left fossae arise the two coronary arteries which supply the substance 

 of the heart. 



The semilunar valves are closed directly after the completion of 

 the contraction of the ventricles. The mechanism involved in this 

 process is similar to that described previously. As the basal portions 



of the different segments are relatively 

 fixed, their free tips are snapped to- 

 gether by the ''breaking" of the ven- 

 tricular jet of blood. The flaps are 

 then held firmly together by the pres- 

 sure existing in the arteries. As is 

 indicated in Fig. 135, this force is di- 

 rected not only in a straight line 

 against their outer surfaces, but also 

 transversely against- their marginal 

 zones. In this way, the under sur- 

 faces of their tips are forced firmly 

 against one another so that a displace- 

 ment and inversion of the segments 

 is quite impossible. Moreover, it is 

 of interest to note that the marginal 

 area of the tip of each flap gives lodgment to a fibrous thickening 

 which rises above the general surface and is adjusted in such a way 

 that it closely fits into the neighboring nodules. In this way, even 

 the most central regions of these arterial orifices are made perfectly 

 secure when the valves are closed. These granular bodies are known 

 as the corpora Arantii.^ 



Fig. 135. — Diagram to Show Posi- 

 tion OF Semilunar Valve Flaps on 

 Closure. 



/, longitudinal section; II, trans- 

 verse section; V, ventricle; A, aorta; 

 FV, fossa of Valsalva; C, corpora 

 arantii. 



CHAPTER XXV 



THE CARDIAC CYCLE (REVOLUTIO CORDIS) 



The Niunber of Cardiac Cycles. — The blood reaches the venous 

 vestibule of the heart under a very low pressure and leaves its arterial 

 orifices under a relatively high pressure. This fact shows that this 

 organ acts as a pump. It develops one of the fundamental attributes 

 of the circulation, namely, the pressure necessary to drive the blood 



1 Named after the Italian anatomist Valsalva of Bologna, born in 1666. 



2 Named after Julius Caesar Aranzi of Bologna, an Italian anatomist, born 

 in 1530. 



