234 PHYSIOLOGY OF THE HEART [CH. XX. 



tight, and, by pulling down the valves, add slightly to the force with 

 which the blood is expelled. 



These statements apply equally to the auriculo-ventricular valves 

 on both sides of the heart ; the closure of both is generally complete 

 every time the ventricles contract. But in some circumstances the 

 tricuspid valve does not completely close, and a certain quantity of 

 blood is forced back into the auricle. This has been called its safety- 

 valve action. The circumstances in which it usually happens are those 

 in which the vessels of the lung are already completely full when the 

 right ventricle contracts, as, e.g., in certain pulmonary diseases, and 

 in very active muscular exertion. In these cases, the tricuspid valve 

 does not completely close, and the regurgitation of the blood may be 

 indicated by a pulsation in the jugular veins synchronous with that 

 in the carotid arteries. 



2. The Semilunar Valves. The commencement of the ventricular 

 systole precedes the opening of the aortic valves by a fraction of a 

 second, as is proved by examining records of the intra ventricular and 

 aortic pressure curves taken simultaneously. The first result of the 

 contraction of the ventricles is the closure of the auriculo-ventricular 

 valves, and as soon as this has been effected the intraventricular 

 pressure begins to rise. It quickly reaches a point at which it equals 

 the aortic pressure, and then exceeds it, and as soon as this pressure 

 difference has been established the aortic valves are opened and blood 

 flows from the ventricle into the aorta. The valves are kept open as 

 long as the intra-ventricular pressure exceeds the aortic, but as soon 

 as the heart has emptied itself, the ventricle begins to relax, its 

 internal pressure consequently begins to fall, and an instant is 

 quickly reached at which it is exceeded by the aortic. The blood, 

 therefore, tends to flow back from the aorta, and in so doing fills up 

 the pockets of the semilunar valves, which have always remained 

 partly filled, and brings them together with a sharp movement. The 

 movements of the valves are therefore effected by the occurrence of 

 differences of pressure upon their two faces. When they meet they 

 completely close the orifice, because their inner edges, which are 

 thinner than the rest of the valves, are brought into apposition 

 and held so by the high pressure acting on their aortic surfaces 

 only. 



The Sounds of the Heart. 



When the ear is placed over the region of the heart, two sounds 

 may be heard at every beat of the heart, which follow in quick 

 succession, and are succeeded by a pause or period of silence. The 

 first or systolic sound is dull and prolonged; its commencement 

 coincides with the impulse of the heart against the chest wall, and 

 just precedes the pulse at the wrist. The second or diastolic sound 



