182 THE CAKDIAC CYCLE. [BOOK i. 



systole. In the systole, the walls of the auricles press towards the 

 auriculo-ventricular orifices, and the auricular appendages are 

 drawn inwards, becoming smaller and paler. During the auricular 

 systole, the ventricles may be seen to become turgid. Then 

 follows, as it were immediately, the ventricular systole, during 

 which the ventricles become more conical. Held between the 

 fingers they are felt to become tense and hard. As the systole 

 progresses, the aorta and pulmonary arteries expand and elongate, 

 the apex is tilted slightly upwards, and the heart twists somewhat 

 on its long axis, moving from the left and behind towards the 

 front and right, so that more of the left ventricle becomes dis- 

 played. As the systole gives way to the succeeding diastole, the 

 ventricles resume their previous form and position, the aorta and 

 pulmonary artery shrink and shorten, the heart turns back 

 towards the left, and thus the cycle is completed. 



In the normal beat, the two ventricles are perfectly synchronous 

 in action ; they contract at the same time and relax at the same 

 time, and the two auricles are similarly synchronous in action. 

 It has been maintained, however, that the synchronism may at 

 times not be perfect. 



Before we attempt to study in detail the several parts of this 

 complicated series of events, it will be convenient to take a rapid 

 survey of what is taking place within the heart during such a cycle. 



109. The cardiac cycle. We may take as the end of the 

 cycle the moment at which the ventricles having emptied their 

 contents have relaxed and returned to the diastolic or resting 

 position and form. At this moment the blood is flowing freely 

 with a fair rapidity, but, as we have seen, at a very low pressure, 

 through the venaB cavse into the right auricle (we may confine 

 ourselves at first to the right side), and since there is now nothing 

 to keep the tricuspid valve shut, some of this blood probably finds 

 its way into the ventricle also. This goes on for some little time, 

 and then comes the sharp, short systole of the auricle, which, 

 since it begins, as we have seen, as a wave of contraction running 

 forwards along the ends of the venae cavse, drives the blood not back- 

 wards into the veins, but forwards into the ventricle ; this result 

 is further secured by the fact that the systole has behind it on the 

 venous side the pressure of the blood in the veins, increasing as 

 we have seen backwards towards the capillaries, and before it the 

 relatively empty cavity of the ventricle in which the pressure 

 is at first very low. By the complete contraction of the auricular 

 walls the complete or nearly complete emptying of the cavity 

 is ensured. No valves are present in the mouth of the superior 

 vena cava, for they are not needed ; and the imperfect Eustachian 

 valve at the mouth of the inferior vena cava cannot be of any 

 great use in the adult, though in its more developed state in 

 the foetus it had an important function in directing the blood of 

 the inferior vena cava through the foramen ovale into the left 



