232 PHYSIOLOGY. 



contract, the mitral and tricuspid valves are tightly pressed together, 

 and regurgitation of blood into the auricles is prevented. Now, as 

 the blood cannot go back into the auricles, it must by the muscular 

 force of the ventricles rush into the pulmonary artery and aorta, 

 respectively. The onset of the blood forces open the semilunar 

 valves of the pulmonary artery and aorta, and exerts a pressure in 

 these arteries partially filled with blood before the new rush of 

 blood sets in. Their walls are necessarily considerably distended. 

 Then the ventricles dilate and at the same time the mitral and tri- 

 cuspid valves open, and the semilunar valves close from the recoil 

 of blood against them. From the time the systole of the ventricles 

 ends to the full distension of the auricles, all the chambers of the 

 heart are in diastole and are being filled with blood. This is the 

 resting of the heart, and is called the pause. 



In the general diastole of the heart, the heart fills itself, the 

 blood passes from the auricles into the ventricles under the influ- 

 ence of the venous blood-pressure. There are three factors which 

 help in the filling of the heart. First, diastolic aspiration of the 

 heart produced by the rapid relaxation of its elastic walls. The 

 second cause is the auricular aspiration produced by the ventricular 

 systole. At the time corresponding exactly to the period of evacu- 

 ation of the heart by a systole a sudden depression ensues in the 

 auricle, a venous aspiration into the auricle, because the ventricles 

 in contracting shorten, but the apex of the heart is not displaced, 

 for the base of the ventricles is pulled towards the apex of the 

 heart; the auriculo-ventricular valves being closed at that time, 

 they are carried along with the ventricle. The floor of the cavities 

 of the auricle is perceptibly lowered and the cavity of the auricle 

 is enlarged. This aspiration of the heart is especially noticeable in 

 the vena cava, but a less marked aspiration is seen in the pul- 

 monary veins.. The third cause in the filling of the heart in a state 

 of general diastole of the heart is the thoracic aspiration. 



PULMONARY CIRCULATION. 



It must be remembered that the right and left auricles fill in 

 the same time; whilst the blood is running into the right auricle 

 from the vena cava, the blood is flowing from the pulmonary veins 

 into the left auricle in the same quantity and with the same 

 rapidity. 



The quantity of blood which the right ventricle sends into the 

 lungs must be the same in quantity as that ejected by the left ven- 



