CHAP, iv.] THE VASCULAR MECHANISM. 229 



organs, which accordingly contract with a sudden sharp 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 ven- 

 tricles 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 displayed. 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. 



127. 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 posi- 

 tion 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 venae cavce 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 for- 

 wards along the ends of the venaa cavse, drives the blood not back- 

 wards into the veins but forwards into the ventricle ; this end 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 



