CH. XVIII.] 



THE HEART. 



2O9 



membrane, or endocardium, which is directly continuous with the 

 internal lining of the arteries and veins. The endocardium is 

 composed of connective tissue with a large admixture of elastic 

 fibres ; its inner surface is covered by endothelium. Here and 

 there unstriped muscular fibres are sometimes found in the tissue 

 of the endocardium. 



Valves. The arrangement of the heart's valves is such that 

 the blood can pass only in one direction (fig. 212). 



The tricuspid valve (5, fig. 208) presents three principal cusps 

 or subdivisions, and the mitral or bicuspid valve has two such 

 portions (6, fig. 209). But in both valves there is between each 



Fig. 2 1 2. Diagram of the circulation through the heart. (Dalton.) 



two principal 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 neighbouring portions, so as 

 to form an annular membrane around the auriculo-ventricular 

 opening, and is fixed to a tendinous ring which encircles the 

 orifice between the auricle and ventricle and receives the inser- 

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

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

 including 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 

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