THE CIRCULATION OF THE BLOOD. 103 



endothelial cells. Here and there unstriped muscular fibres are some- 

 times found m the tissue of the endocardium. 



Valves of the Heart. The arrangement of the heart's valves is 

 such that the blood can pass only in one direction (Fig. 94). 



The tricuspid valve (5, Fig. 89) presents three principal cusps or 

 subdivisions, and mitral or bicuspid valve, because it has two such por- 

 tions (6, Fig. 90). But in both valves there is between each two prin- 

 cipal portions a smaller one ; so that more properly, the tricuspid may 

 be described as consisting of six, and the mitral of four, portions. Each 

 portion is of triangular form, its base is continuous with the bases of the 

 neighboring portions, so as to form an annular membrane around the 

 auriculo-ventricular opening, and is fixed to a tendinous ring which en- 



FIG. 94. -Diagram of the circulation through the heart (Dalton). 



circles the orifice between the auricle and ventricle and receives the 

 insertions of the muscular fibres of both. In each principal cusp may 

 be distinguished a central part, extending from base to apex, and includ- 

 ing about half its width. It is thicker, and much tougher than the 

 border-pieces or edges. 



While the bases of the cusps of the valves are fixed to the tendinous 

 rings, their ventricular surface and borders are fastened by slender ten- 

 dinous fibres, the chordcB tendinece, to the internal surface walls of the 

 ventricles, the muscular fibres of which project into the ventricular cavity 

 in the form of bundles or columns the columnce carnece. These columns 

 are not all alike, for while some are attached along their whole length 

 on one side, and by their extremities, others are attached only by their 



