114 HAND-BOOK OF PHYSIOLOGY. 



wall of the ventricle is brought by its contraction nearer the auriculo- 

 ventricular orifice, the musculi papillares more than compensate for this 

 by their own contraction holding the cords tight, and, by pulling down 

 the valves, adding slightly to the force with which the blood is expelled. 



What has been said applies equally to the auriculo-ventricular valves 

 on both sides of the heart, and of both alike the closure is generally com- 

 plete every time the ventricles contract. But in some circumstances the 

 closure of the tricuspid valve is not complete, and a certain quantity of 

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

 valve action of this valve. The circumstances in which it usually happens 

 ^re those in which the vessels of the lung are already full enough when 

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

 very active exertion, and in great efforts. 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. 



Function of the Semilunar Valves. The arterial or semilunar 

 valves are forced apart by the out-streaming blood, with which the con- 

 tracting ventricle dilates the large arteries. The dilation of the arteries 

 is, in a peculiar manner, adapted to bring the valves into action. The 

 lower borders of the semilunar valves are attached to the inner surface of 

 a tendinous ring, which is, as it were, 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 this ring is tough, and does not admit of 

 extension under such pressure as it is commonly exposed to; the valves 

 are equally inextensile, being, as already mentioned, formed of tough, close- 

 textured, fibrous tissue, with strong interwoven cords, and covered with 

 endocardium. Hence, when the ventricle propels blood through the ori- 

 fice and into the canal of the artery, the lateral pressure which it exercises 

 is sufficient to dilate the walls of the artery, but not enough to stretch in an 

 equal degree, if at all, the unyielding valves and the ring to which their 

 lower borders are attached. The effect, therefore, of each such propul- 

 sion of blood from the ventricle is, that the wall of the first portion of 

 the artery is dilated into three pouches behind the valves, while the free 

 margins of the valves are drawn inward toward its centre (Fig. 98, B). 

 Their positions may be explained by the diagrams, in which the continu- 

 ous lines represent a transverse section of the arterial walls, the dotted 

 ones the edges of the valves, firstly, when the valves are nearest to the 

 walls (A), and, secondly, when, the walls being dilated, the valves are 

 drawn away from them (B). 



This position of the valves and arterial walls is retained so long as the 

 ventricle continues in contraction: but, as soon as it relaxes, and the di- 

 lated arterial walls can recoil by their elasticity, the blood is forced back- 

 ward toward the ventricles as onward in the course of the circulation. 



