MOVEMENTS OF THE HEART. 183 



or sigmoid valves of the left side, present no difference from 

 the valves at the orifice of the pulmonary artery. They are 

 situated 'at the aortic orifice. 



The physiological anatomy of the tricuspid and mitral 

 valves may be studied, by cutting away the auricles so as to 

 expose the auriculo-ventricular openings, introducing a pipe 

 into the pulmonary artery and aorta, after destroying the 

 semilunar valves, and then forcing water into the ventricles 

 by a syringe or from a hydrant. In this way the play of the 

 valves will be beautifully exhibited. 



We can study the action of the semilunar valves, by 

 cutting away enough of the ventricles to expose them, and 

 forcing water into the vessels. These experiments give an 

 idea of the immense strength of the valves; for they can 

 hardly be ruptured by a force which is not sufficient to rup- 

 ture the vessels themselves. 



Movements of the Hea/rt. 



In studying the phenomena which accompany the action 

 of the heart, we shall follow the course of the blood, begin- 

 ning w r ith it as it flows from the vessels into the auricles. 

 The dilatation of the cavities of the heart is called the diastole, 

 and their contraction the systole. When these terms are 

 used without any qualification, they are understood as refer- 

 ring to the ventricles ; but they are also applied to the action 

 of the auricles, as the auricular diastole or systole, which, as 

 we shall see, is distinct from the action of the ventricles. 



A complete revolution, so to speak, of the heart consists 

 in the filling and emptying of all its cavities, during which 

 they experience an alternation of repose and activity. As 

 these phenomena occupy, in many warm-blooded animals, a 

 period of time less than one second, it will be appreciated 

 that the most careful study is necessary in order to ascertain 

 their exact relations to each other. When the heart is ex- 

 posed in a living animal, the most prominent phenomenon 



