AUKIOULO-VENTRICULAR VALVES. 47 



anatomy of the valves which guard the orifices of the ventricles gives an idea of their 

 function ; but we have yet to consider the precise mechanism by which they are opened 

 and closed and the way in which regurgitation is prevented. 



In man and the warm-blooded animals, there are no valves at the orifices by which 

 the veins open into the auricles. As has already been seen, compared with the ventri- 

 cles, the force of the auricles is insignificant ; and it has farthermore been shown by ex- 

 periment that the ventricles may be filled with blood and the circulation continue, when 

 the auricles are entirely passive. Although the orifices are not provided with valves, 

 the circular arrangement of the fibres about the veins is such, that during the contrac- 

 tion of the auricles the openings are considerably narrowed, and regurgitation cannot 

 take place to any great extent. The force of the blood flowing into the auricles like- 

 wise offers an obstacle to its return. There is really no valvular apparatus which oper- 

 ates to prevent regurgitation from the heart into the veins; for the valvular folds, 

 which are so numerous in the general venous system, and particularly in the veins of 

 the extremities, do not exist in the venae cavas. The continuous flow of blood from the 

 veins into the auricles, the feeble character of the auricular contractions, the arrange- 

 ment of the fibres around the orifices of the vessels, and the great size of the auriculo- 

 ventricular openings, are conditions which provide sufficiently for the flow of blood into 

 the ventricles. 



Action of the Auricula- Ventricular Valves. After the ventricles have become com- 

 pletely distended by the auricular systole, they take on their contraction, which, it will 

 be remembered, is very many times more powerful than the contraction of the auricles. 

 They have to force open the valves which close the orifices of the pulmonary artery and 

 aorta and empty their contents into these vessels. To accomplish this, at the moment of 

 the ventricular systole, there is an instantaneous and complete closure of the auriculo- 

 ventricular valves, leaving but one opening through which the blood can pass. That 

 these valves close at the moment of contraction of the ventricles is demonstrated by the 

 experiments of Chauveau and Faivre, who introduced the finger through an opening 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 pro- 

 duced in great 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 slight- 

 ly 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 semilunar 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 approxi- 

 mated that not a drop passes between them ; but, when the pressure is considerable, a 

 certain quantity of fluid passes the tricuspid valve. There is, indeed, a certain amount 

 of insufficiency at the right auriculo-ventricular orifice, which does not, exist on the 

 opposite side. 



The fact just noted was first pointed out by Mr. T. W. King, and is called by him the 

 " safety-valve function of the right ventricle." The advantage of this slight insufficiency 

 is apparent on a little reflection. The right ventricle sends its blood to the lungs, where, 

 in order to facilitate the respiratory processes, the walls of the capillaries are very thin. 

 The lungs themselves are exceedingly delicate, and an effusion of blood, or considerable 

 congestion, would be liable to be followed by serious consequences. To prevent this, the 

 right ventricle is not permitted to exert all its force, under all circumstances, upon the 

 blood going into the pulmonary artery ; but, when the action of the heart is exaggerated 

 from any cause, the lungs are relieved by a slight regurgitation, which takes place through 



