268 THE MECHANICS OF THE HEART 



which, as the name indicates, are tendinous cords extending from 

 here to the overlying valve flaps. 



The Auriculoventricular Valves. — It has been stated above that 

 the blood flows through the heart in a perfectly definite direction, 

 because the contraction of the ventricles does not take place until the 

 contraction of the auricles has been completed, and because the orifices 

 connecting the different chambers of this organ are opened and 

 closed in perfect harmony with the activity of the cardiac muscle. 

 There are really two ways in which the cardiac orifices could be closed : 

 namely, by heavy rings of muscle tissue which by their sphincter-like 

 action obliterate the passage in the manner of the diaphragm of a 

 photographic camera, or by membranous flaps which, in the manner of 

 a door, swing directly across the openings. Clearly, the closure of an 

 orifice by a layer of circular musculature is an action which requires 

 power and, therefore, necessitates the expenditure of a considerable 

 amount of energy. If this mechanism were actually in use in our 

 heart, it would mean that the pressure developed by this organ would 

 have to be apportioned in part to the closure of its orifices, and in part 

 to the blood as driving force. For this reason, the use of valves must 

 be considered as a much more economical means, inasmuch as it does 

 not necessitate a division of the cardiac energy. The different valve 

 flaps are moved into place passively by the relative degrees of pressure 

 upon their two surfaces, and hence, all the power developed by the 

 heart may be directed to the single purpose of propelling the blood. 

 In this way, the closure of the valves is accomplished, so to speak, 

 incidentally in the course of the general muscular contraction. 



The auriculoventricular openings are large and are especially 

 adapted for a quick transfer of blood. The left is oval in shape and 

 smaller than the right which possesses a rounded triangular outline. 

 Both orifices are surrounded by fibrous rings which are connected 

 with the mass of the fibrocartilaginous tissue situated at the auriculo- 

 ventricular junction. The different valve flaps are composed of double 

 folds of endocardium, strengthened by fibrous tissue and containing a 

 few elastic fibers and muscle cells. The latter are arranged radially 

 and are connected with the auricular musculature. The basal por- 

 tions of the fiaps are fastened to the walls of the orifice, while their 

 tips and thin margins are free and project far into the cavity. 



The left valve, known as the mitral, is composed of two triangular 

 flaps of unequal size, while the right, or tricuspid, consists of three 

 flaps. Both valves yield solely in a downward direction and on closure 

 assume a position transversely across the opening. A perfect approxi- 

 mation of the different flaps is made possible, on the one hand, by the 

 muscle tissue forming the wall of the orifice, and, on the other, by the 

 chordae tendinese'with which their lower surfaces are connected. Ob- 

 viously, the contraction of the former gives a certain firmness to the 

 frame in which the valve flaps are hung so that their basal portions 

 become flxed, while their tips attain a wide range of movement. In 



