38 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



less than one second, it will be appreciated that the most careful study is ne- 

 cessary 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 practically the same as in the human subject ; and 

 although the exposure of the heart by opening the chest modifies somewhat 

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

 other in their natural order and present essentially their normal characters. 

 Having opened the chest, keeping up artificial respiration, the heart, en- 

 veloped in its pericardium, is observed, 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 contrac- 

 tion of the auricles, these cavities are continually receiving blood, on the right 

 side from the general system, by the venae cavae, and on the left side from the 

 lungs, by the pulmonary veins. This continues until the cavities of the au- 

 ricles are completely filled, the blood coming in by a steady current; and 

 during the repose of the heart, the blood is also flowing through the auriculo- 

 ventricular orifices into the ventricles. When the auricles have become fully 

 distended, they contract quickly and with considerable power (the auricular 

 systole), and force the blood into the ventricles, producing complete diastole 

 of these cavities. During this contraction, the blood not only ceases to flow 

 in from the veins, but some of it is regurgitated, as the orifices by which the 

 vessels open into the auricles are not provided with valves. The size of the 

 auriculo- ventricular orifices is one reason why the greater portion of the blood 

 is made to pass into the ventricles; and farthermore, during the auricular 

 systole, the muscular fibres which are arranged around the orifices of the 

 veins constrict them to a certain extent, which tends to diminish the reflux 

 of blood. There can be no doubt that some regurgitation takes place from 

 the auricles into the veins, but this prevents the possibility of over-distention 

 of the ventricles. 



It has been shown that the systole of the auricles is not immediately neces- 

 sary to the performance of the circulation ; and the contractility of the auri- 

 cles may be temporarily exhausted by repeated and prolonged stimulation, 

 the ventricles continuing to act, keeping up the circulation of blood. 



Action of the Ventricles. Immediately following the contraction of the 

 auricles, by which the ventricles are completely distended, there is contrac- 

 tion of the ventricles. This is the chief active operation performed by the 

 heart and is generally spoken of as the systole. The contraction of the ven- 

 tricles is very much more powerful than that of the auricles. By their ac- 

 tion, the blood is forced from the right side to the lungs, by the pulmonary 

 artery, and from the left side to the general system, by the aorta. Eegurgita- 



