274 TEXT-BOOK OF PHYSIOLOGY. 



lar walls quickly relax and remain at rest until the close of the next auricular 

 contraction. The summit of the ventricular tracing generally spoken of as 

 the plateau presents a series of elevations and depressions as stated in a 

 foregoing paragraph. 



A comparison of the two traces shows that between the close of the 

 auricular and the beginning of the ventricular systole there is a slight pause 

 known as the inter systolic pause (Chauveau). The tracings also show that 

 between the close of the ventricular contraction and the beginning of the 

 succeeding auricular contraction there is a period during which the whole 

 heart is at rest and the cavities filling with blood. 



For the purpose of obtaining the time of all these events, the recording 

 surface was divided into equal spaces by vertically drawn lines. The rate 

 of movement of the surface was such that each division corresponded to 

 one-tenth of a second. The record thus indicates that the auricular con- 

 traction lasted approximately 0.2 second, the ventricular contraction 0.4 

 second, and the pause 0.4 to 0.6 second. 



From similar experiments made on other animals, e.g., the dog, similar 

 results have been obtained; but by reason of the employment of more 

 sensitive and more quickly responsive tambours, the curve of the auricular 

 contraction exhibits variations not recorded by the forms of tambour used 

 in earlier experiments. Reference to these variations will be alluded to in 

 subsequent paragraphs. The results obtained by recent observers now 

 generally accepted are in accord with the results obtained by Chauveau and 

 Marey by means of their cardiac tambours as shown in Fig. 125. 



From the foregoing facts it is apparent that with the relaxation of the 

 auricular walls, blood at once flows from the venae cavae and the pulmonary 

 veins into the auricular cavities and continues so to do throughout the entire 

 auricular diastole. With the relaxation of the ventricular walls, however, the 

 blood that has accumulated in the auricles up to this time, or its equivalent 

 coming from the venae cavae and pulmonary veins, now flows into the ventricles 

 until they are nearly filled. Before they are filled, however, the auricular dias- 

 tole comes to an end, the auricular walls again contract and force their con- 

 tained blood into the ventricles and thus rapidly complete the filling. The 

 ventricular systole immediately follows, during which the blood is driven into 

 the pulmonary artery and aorta. This having been accomplished, the ven- 

 tricles relax, and the blood that has been accumulating in the auricles begins 

 to flow into the ventricles, after which the same series of events follows as in 

 the previous cycle. 



The Action of the Valves During the Cycle. As previously stated, 

 the forward movement of the blood is permitted and regurgitation prevented 

 by the alternate action of the semilunar and the auriculo-ventricular valves. 

 As a point of departure for a consideration of the action of the valves and 

 their relation to the systole and diastole of the heart, the close of the ventricu- 

 lar systole may be conveniently selected. 



At this moment, if the blood is not to be returned to the ventricles, the 

 semilunar valves must be instantly and completely closed. This is accom- 

 plished in the following manner: During the outflow of blood from the 

 ventricles the valves are pushed outward toward the walls of the vessels, 

 though not coming into contact with them, for behind them are the pouches 



