112 HAND-BOOK OF PHYSIOLOGY. 



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 backward; 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 ven- 

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

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

 plete septum between it and the auricle. 



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 quantity of blood 

 remains. The shape of both ventricles during systole undergoes an alter- 

 ation, the left probably not altering in length but to a certain degree in 

 breadth, the diameters in the plane of the base being diminished. The 

 right ventricle does actually shorten to a small extent. The systole has 

 the effect of diminishing the diameter of the base, especially in the plane 

 of the auriculo-ventricular valves; but the length of the heart as a whole 

 is not altered. (Ludwig.) During the systole of the ventricles, too, the 

 aorta and pulmonary artery, being filled with blood by the force of the 

 ventricular action against considerable resistance, elongate as well as ex- 

 pand, and the whole heart moves slightly toward the right and forward, 

 twisting on its long axis, and exposing more of the left ventricle ante- 

 riorly than is usually in front. When the systole ends the heart resumes 

 its former position, rotating to the left again as the aorta and pulmonary 

 artery contract. 



Functions of the Auriculo-Ventricular Valves. The disten- 

 sion of the ventricles wiifi blood continues throughout the whole period 

 of their diastole. The auriculo-ventricular valves are gradually brought 

 into play by some of the blood getting behind the cusps and floating them 

 up; and by the time that the diastole is complete, the valves are no doubt 

 in apposition, the completion of this being brought about by the reflex 

 current caused by the systole of the auricles. This elevation of the au- 



