llfi HANDBOOK OF PHYSIOLOGY. 



precedes the beat of the heart against the side of the chest. At this 

 period the whole heart is in a passive state, the walls of both auricles and 

 ventricles are relaxed, and their cavities are becoming dilated. The 

 auricles are gradually filling with blood flowing into them from the 

 veins; and a portion of this blood passes at once through them into the 

 ventricles, the opening between the cavity of each auricle and that of 

 its corresponding ventricle being, during all the pause, free and patent. 

 The auricles, however, receiving more blood than at once passes through 

 them to the ventricles, become, near the end of the pause, fully distended; 

 and at the end of the pause, they contract and expel their contents into 

 the ventricles. 



The contraction of the auricles is sudden and very quick; it com- 

 mences at the entrance of the great veins into them, and is thence pro- 

 pagated towards the auriculo-ventricular opening; but the last part 

 which contracts is the auricular appendix. The effect of this contraction 

 of the auricles is to quicken the flow of blood from them into the ven- 

 tricles; the force of their contraction not being sufficient under ordinary 

 circumstances to cause any back-flow in the veins. The reflux of blood 

 into the great veins is moreover resisted not only by the mass of blood in 

 the veins and the force with which it streams into the auricles, but also 

 by the simultaneous contraction of the muscular coats with which the 

 large veins are provided near their entrance into the auricles. Any slight 

 regurgitation from the right auricle is limited also by the valves at the 

 junction of the subclavian and internal jugular veins, beyond which the 

 blood cannot move backwards; and the coronary vein is preserved from 

 it by a valve at its mouth. 



In birds and reptiles regurgitation from the right auricle is prevented 

 by valves placed at the entrance of the great veins. 



During the auricular contraction the force of the blood propelled into 

 the ventricle is transmitted in all directions, but being insufficient to 

 separate the semilunar valves, it is expended in distending the ventricle, 

 and, by a reflux of the current, in raising and gradually closing the au- 

 riculo-ventricular valves, which, when the ventricle is full, form a com- 

 plete septum between it and the auricle. 



2. Action of the Ventricles. The blood which is thus driven, by 

 the contraction of the auricles, into the corresponding ventricles, being 

 added to that which had already flowed into them during the heart's 

 pause, is sufficient to complete their diastole. Thus distended, they 

 immediately contract: so immediately, indeed, that their systole looks as 

 if it were continuous with that of the auricles. The ventricles contract 

 much more slowly than the auricles, and in their contraction probably 

 always thoroughly empty themselves, differing in this respect from the 

 auricles, in which, even after their complete contraction, a small quan- 



