178 HANDBOOK OF PHYSIOLOGY. 



opening. Fibrous tissue also exists at the origins of the pulmonary 

 artery and aorta. 



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. The fibres of the auricles are, however, quite separate 

 from those of the ventricles, the bond of connection between them 

 being only the fibrous tissue of the auriculo-ventricular openings. 



The minute structure of the striated muscular fibres of the heart 

 has been already described (p. 88). 



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

 transparent layer of pericardium, so its cavities are lined by a smooth 



Fig. 151. Diagram of the circulation through the heart (Dalton) 



and shining 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; 

 and on its inner surface is laid down a single tesselated layer of flat- 

 tened endothelial cells. 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. 151). 



The tricuspid valve (5, fig. 147) presents three principal cusps or sub- 

 divisions, and the mitral or bicuspid valve has two such portions (6, fig. 

 148). 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 neighbor- 



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