234 



PHYSIOLOGY OF TIIE HEART. 



[CH. xx. 



The musculi papillares prevent the auriculo-ventricular valves 

 from being everted into the auricle. For the chordae tendineae 

 might allow the valves to be pressed back into the auricle, were 

 it not that when the wall of the ventricle is brought by its con- 

 traction nearer the auriculo - ventricular orifice, the musculi 

 papillares more than compensate for this by their own contraction ; 

 they hold the chords 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 circum- 

 stances 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 exer- 

 tion. In these cases, the 

 tricuspid valve does not 

 completely close, and the 

 regurgitation of the blood 

 may be indicated by a pul- 

 sation in the jugular veins 

 synchronous with that in 

 the carotid arteries. 

 2. The Semilunar Valves. It has been found that the com- 

 mencement of the ventricular systole precedes the opening of the 

 semilunar valves by a fraction of a second. This shows that the 

 intraventricular pressure does not exceed the arterial pressure 

 until the systole has actually begun, for the opening of the valves 

 takes place at once when there is a distinct difference in favour 

 of the intraventricular over the arterial pressure, and they continue 

 open only as long as this difference continues. When the arterial 

 exceeds the intraventricular pressure, there is a tendency of the 

 blood to flow back to the heart, and this closes the valves. The 

 dilatati6n of the arteries is, in a peculiar manner, adapted to bring 

 this about. The lower borders of the semilunar valves are attached 

 to the inner surface of the tendinous ring, which is inlaid at the 

 orifice of the artery, between the muscular fibres of the ventricle 

 and the elastic fibres of the walls of the artery. The tissue of 



Fig. 234. Sections of aorta, to show the action of 

 the semilunar valves. A is intended to show 

 the valves, represented by the dotted linrs, 

 lying near the arterial walls, represented by 

 the continuous outer line. B (after Hunter) 

 shows the arterial wall distended into three 

 pouches (a), and drawn away from the valves, 

 which are straightened into the form of an 

 equilateral triangle, as represented by the 

 dotted lines. 



