ACTION OF THE VALVES. 645 



in front, a distension of the elastic walls of the arteries themselves. 

 But the ventricles, having contracted, after a short pause begin to 

 dilate again, and then the columns of blood in the two great arteries, 

 reacted upon by the recoil of the distended and elongated coats of 

 those vessels, would flow back towards the ventricles, if the segments 

 of the semilunar valves did not speedily open out across the mouths of 

 those vessels. The blood first getting into the pouches, or sinuses, of 

 Valsalva behind the valves, an action which is facilitated by the pro- 

 jecting corpora Arantii, then presses on the whole arterial surface of 

 the segments, and so regurgitation into the ventricles is prevented. 

 The force which thus acts upon the two columns of blood, to close the 

 semilunar valves, is the resilience of the coats of the previously dis- 

 tended arteries ; but it is itself derived from the heart's action, which 

 has caused distension of the vessels. As the muscular walls of the 

 ventricles relax, and their cavities dilate, the tricuspid and mitral 

 valves reopen, and blood passing from the auricles, begins to fill them 

 again. In the meantime, the auricles themselves have been dilating 

 to the point of distension, and again, at that moment, the actions 

 of the heart necessary to the circulation recommence, viz., the rapid 

 contraction of the auricles, the complete distension of the ventricles, 

 and the sudden contraction of these cavities. 



A complete action of the heart consists of a single systole, and dia- 

 stole of its auricles and ventricles. The period occupied in such an 

 action commences with the systole of the auricles, includes the systole 

 of the ventricles, and terminates at the perfect diastole of the auricles, 

 when these cavities are fully distended and ready to perform their 

 systole again. During this period of a complete cardiac act, the auri- 

 cles contract quickly, whilst the ventricles contract a little more slowly; 

 but the auricles dilate slowly, whilst the ventricles dilate more quickly. 

 Hence (see the Table at p. 650), the systole and diastole of the two 

 sets of cavities, occupy different parts of the entire period of a single 

 cardiac action or beat. 



The diastole of both the auricles and ventricles, has been supposed 

 to depend on an active contraction of particular sets of their muscular 

 fibres ; but it is now generally regarded as a passive phenomenon of 

 dilatation, dependent on the rest or relaxation of all their fibres. 



The systole of the auricles consists of a progressive contraction, 

 which spreads from the orifices of the great veins towards the auricu- 

 lo-ventricular openings, the auricular appendices acting last ; the 

 auricles, therefore, contract towards the base of the ventricles. The 

 systole of the ventricles is not only somewhat slower, but is more 

 uniform than that of the auricles, being simultaneous through every 

 part of their walls, the total result being likewise to draw them 

 towards their base, i. e., towards the orifices of the great arterial 

 trunks. By this action, the ventricular portion of the heart is 

 shortened, rendered thicker from back to front, and more convex on 

 its anterior surface; it is also widened a little posteriorly (Budge); 

 at the same time, it becomes very firm and hard, like any other 

 contracted muscular mass. Owing to the elongation of the great 

 arterial trunks, at the moment of their distension by the blood, the 



