FUNCTION OF THE YALYES. 125 



the end of the simultaneously contracting fleshy column is 

 drawn away from it, and the cords are held tight. 



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 : and, 

 since this may be advantageous, by preventing the over- 

 filling of the vessels of the lungs, it has been called the 

 safety-valve action of this valve (Hunter, Wilkinson King). 

 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 exertion, and in great efforts. In 

 these cases, perhaps, because the right ventricle cannot 

 contract quickly or completely enough, the tricuspid valve 

 does not completely close, and the regurgitation of blood 

 may be indicated by a pulsation in the jugular veins syn- 

 chronous with that in the carotid arteries. 



The arterial or semilunar valves are, as already said, 

 brought into action by the pressure of the arterial blood 

 forced back towards the ventricles, when the elastic walls 

 of the arteries recoil after being dilated by the blood pro- 

 pelled into them in the previous contraction of the ventricle. 

 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 

 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, 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 inter- 

 woven cords, and covered with endocardium. Hence, when 



