98 CIRCULATION 



the auricle is impossible. The semilunar valves form a guard 

 against the return of the blood to the ventricle at the pulmo- 

 nary and aortic openings. These valves are forced open during 

 the ventricular contraction by the blood which passes through 

 them to distend the elastic walls of the large arteries. The 

 pressure of the blood under the elastic recoil is sufficient to 

 throw the cusps of the valves into action. The corpora Arantii 

 are useful in making a perfect closure of the valve, although 

 not absolutely essential. A part of the weight of this pressure 

 is borne by the thick ventricular wall, which forms a ring from 

 the outer edge of which the arteries spring, while the valves 

 are attached to the inner edge. 



Venous Pulse. Under some circumstances the tricuspid 

 valve does not entirely close, but allows a certain amount of 

 regurgitation of blood. This occurs in conditions of disease or 

 of violent exercise, when the lung capillaries are overcharged 

 with blood. The leakage of the valve is conservative, and 

 relieves the pressure upon the delicate capillaries of the pulmo- 

 nary system. A pulsation in the jugular vein, in which the 

 chief elevation is synchronous with the ventricular systole, 

 indicates such a regurgitation. It must not be confused with 

 a communicated venous pulse due simply to the proximity 

 of some large artery; nor with a venous pulse due to a trans- 

 mission of the arterial pulse through widened arterioles and 

 capillaries; nor with a venous pulse in the jugular which depends 

 upon variations of pressure in the auricle. The latter kind 

 of venous pulse is of the greatest importance in an analysis 

 of cardiac events. It manifests three distinct elevations or 

 positive waves, separated by three depressions or negative 

 waves. The first elevation corresponds to the systole of the 

 auricle. The second is synchronous with the ventricular systole, 

 and due, perhaps, to a sudden bulging of the auriculo ventricular 

 valve into the auricle. The cause of the third positive wave 

 is not so clear. It is said to be due to the inflow of the venous 

 blood. Its irregularity is occasioned by the muscular relaxation 

 of the ventricle, and the return, therefore, of the base of the 

 ventricle to its diastolic position. The auriculoventricular 

 valve opens immediately afterward and the accumulated blood 

 in the auricle passes into the ventricle causing a sudden fall 

 of pressure in auricle and vein. 



