44 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



the moment of contraction of the ventricles, was demonstrated by the experi- 

 ments of Chauveau and Faivre, who introduced the finger through an open- 

 ing into the auricle and actually felt the valves close at the instant of the 

 ventricular systole. This tactile demonstration, and the fact that the first 

 sound of the heart, which is produced in part by the closure of the auriculo- 

 ventricular valves, is synchronous with the ventricular systole, leave no doubt 

 as to the mechanism of the closure of these valves. It is probable that as 

 the blood flows into the ventricles, the valves are slightly floated out, but they 

 are not closed until the ventricles contract. 



If a bullock's heart be prepared by cutting away the auricles so as to 

 expose the mitral and tricuspid valves, securing the nozzles of a double 

 syringe in the pulmonary artery and aorta after having destroyed the semi- 

 lunar valves, and if fluid be injected simultaneously into both ventricles, the 

 play of the valves will be exhibited. The mitral valve effectually prevents 

 the passage of fluid, its edges being so accurately adapted that not a drop 

 passes between them ; but when the pressure is considerable, a certain quan- 

 tity of fluid passes the tricuspid valve (T. W. King). There is, indeed, a 

 certain degree of insufficiency of the tricuspid valve, which does not exist on 

 the opposite side ; but it is very questionable whether there can be sufficient 

 force exerted by the right ventricle to produce regurgitation of blood at the 

 right auriculo-ventricular orifice. 



Action of the Aortic and Pulmonic Valves. The action of the semilunar 

 valves is nearly the same upon both sides. In the intervals of the ventricular 

 contractions, they are closed and prevent regurgitation of blood into the ven- 

 tricles. The systole, however, overcomes the resistance of these valves and 

 forces the contents of the ventricles into the arteries. During this time, the 

 valves are applied, or nearly applied, to the walls of the vessel ; but so soon 

 as the ventricles cease their contraction, the constant pressure of the blood, 

 which is very great, closes the openings. 



The action of the semilunar valves can be studied by cutting away a por- 

 tion of the ventricles in the heart of a large animal, securing the nozzles of a 

 double syringe in the aorta and pulmonary artery and forcing water into the 

 vessels. It has been observed that while the aortic semilunar valves oppose 

 the passage of the liquid so effectually that the aorta may be ruptured before 

 the valves will give way, a certain degree of insufficiency exists, under a high 

 pressure, at the orifice of the pulmonary artery (Flint, 1864). A slight in- 

 sufficiency of the pulmonic valves was observed by John Hunter, in 1794. It 

 is not probable, however, that the pressure of blood in the pulmonary artery 

 is ever sufficient to produce regurgitation when the valves are normal. 



It is probable that the corpuscles of Arantius, which are situated in the 

 middle of each valvular curtain, assist in the accurate closure of the orifice. 

 The sinuses of Valsalva, situated in the artery behind the valves, are regarded 

 as facilitating the closure of the valves by allowing the blood to pass easily 

 behind them. 



Sounds of the Heart. The appreciable phenomena which attend the 

 heart's action are connected with the systole of the ventricles. It is this 



