en. XVIIL] 



THE HEART 



211 



The muscular fibres of each auricle are in part continuous with 

 those of the other, and partly separate; and the same remark 

 holds true for the ventricles. Some muscular fibres also pass across 



FIG. L'L'O.Nt't \vurk of muscular fibres from the heart of a pig. The nuclei are well shown, x 450. 



(Klein and Noble Smith.) 



the tendinous ring which separates each auricle from the correspond- 

 ing ventricle. 



Endocardium. As the heart is clothed on the outside by the 

 epicardium, so its cavities are lined by a smooth membrane, the 

 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 muscular fibres are 

 sometimes found in the tissue 

 of the endocardium. 



Valves. The arrange- 

 ment of the heart's valves is 

 such that the blood can pass 

 only in one direction (fig. 

 221). 



The tricuspid valve (5, fig. 

 217) presents three principal 

 cusps or subdivisions, and the 

 mitral or bicuspid valve has 

 two such portions (6, fig. 218). 

 But in both valves there is 

 between each 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 



FIG. 221. Diagram of the circulation through the 

 heart. (Dalton.) 



