40 CIRCULATION" OF THE BLOOD. 



FIG. 17. Valves of the heart. (Bonamy and Beau.) 



1, Eight auriculo- ventricular orifice, closed by the tricuspid valve; 2, fibrinous ring; 3, left auriculo-ventricular 

 orifice, closed by the mitral valve; 4, fibrinous ring ; 5, aortic orifice and valves: 6, pulmonic orifice and valves ; 

 7, 8, 9, muscular fibres. 



Movements of the Heart. 



In studying the phenomena which accompany the action of the heart, we shall fol- 

 low the course of the blood, beginning with it as it flows from the vessels into the auri- 

 cles. The dilatation of the cavities of the heart is called the diastole, and the contrac- 

 tion of the heart, the systole. When these terms are used without any qualification, 

 they are understood as referring 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 of the heart consists in the filling and emptying of all its cavi- 

 ties, during which they experience an alternation of repose and activity. As these phe- 

 nomena 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 exposed in a living animal, the most 

 prominent phenomenon is the alternate contraction and relaxation of the ventricles ; 

 but this is only one of the operations of the organ. In all the mammalia, the anatomy 

 and action of the vascular system are to all intents and purposes the same as in the hu- 

 man subject ; and, although the exposure of the heart by opening the chest modifies some- 

 what the force and frequency of its pulsations, the various phenomena follow each other 

 in their natural order and present essentially their normal characters. The operation of 

 exposure of the heart may be performed on a living animal without any great difficulty ; 

 and. if we simply take care to keep up artificial respiration, the action of the heart will 

 continue for a considerable time. We may keep the animal quiet by the administration 

 of ether or by poisoning with woorara, the latter agent acting upon the motor nerves 

 but having no effect upon the heart. Having opened the chest, we see the heart, envel- 

 oped in its pericardium, contracting regularly ; and, on slitting up and removing this 

 covering, the various parts are completely exposed. The right ventricle and auricle and 

 a portion of the left ventricle can be seen without disturbing the position of the parts ; 

 but the greater part of the left auricle is concealed. As both auricles and ventricles act 

 together, the parts of the heart which are exposed are sufficient for purposes of study. 



Action of the Auricles. Except the short time occupied in the contraction of the 

 auricles, these cavities are continually receiving blood on the right side from the system, 

 by the vense cavse, and on the left side from the lungs, by the pulmonary veins. This 



