118 HANDBOOK OF PHYSIOLOGY. 



longitudinal fibres the ventricle would shorten on systole ; but there are 

 both. The tendency to alter in length is thus counter-balanced, and the 

 whole force of the contraction is expended in diminishing the cavity of 

 the ventricle ; or, in other words, in expelling its contents. 



On the conclusion of the systole the ventricular walls tend to expand 

 by virtue of their elasticity, and a negative pressure is set up, which 

 tends to suck in the blood. This negative or suctional pressure on the 

 left side of the heart is of the highest importance in helping the pul- 

 monary circulation. It has been found to be equal to 23 mm. of mer- 

 cury, and is quite independent of the aspiration or suction power of the 

 thorax, which will be described in the chapter on Kespiration. 



Function of the Musculi Papillares. The special function of the 

 musculi papillares is to prevent the auriculo-ventricular valves from be- 

 ing everted into the auricle. For the chordas 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 contraction nearer the auriculo-ventric- 

 ular 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 

 complete 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 safe- 

 ty-valve action of this valve. The circumstances in which it usually 

 happens are 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 exertions, 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 synchro- 

 nous with that in the carotid arteries, 



(2) Of the Semilunar Valves. The arterial or semilunar valves are 

 forced apart by the out-streaming blood, with which the contracting 

 ventricle dilates the large arteries. The dilatation 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 the 

 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 

 mainly of tough, close-textured, fibrous tissue, with strong interwoven 

 cords. Hence, when the ventricle propels blood through the orifice and 

 into the canal of the artery, the lateral pressure which it exercises is 



