THE CIRCULATION OF THE BLOOD. 



263 



The Endocardium. The cavities of both the right and left sides of the 

 heart are lined by a thin, firm connective-tissue membrane, closely adherent 

 to the muscle-tissue, termed the endocardium. It contains also elastic fibers 

 and smooth muscle-fibers. Its entire surface is covered with a layer of 

 polygonal endothelial cells. This membrane serves partially to resist undue 



FIG. 116 FIG. 117. 



FIG. 116. INTERIOR OF RIGHT AURICLE AND VENTRICLE, EXPOSED BY THE REMOVAL OF 

 A PART OF THEIR WALLS, i, Superior vena cava; 2, inferior vena cava; 2', hepatic veins; 3, $',$". 

 inner wall of right auricle; 4, 4, cavity of right ventricle; 4', papillary muscle; 5, 5', 5", flaps of 

 tricuspid valve; 6, pulmonary artery in the wall of which a window has been cut; 7, on aorta 

 near the ductus arteriosus; 8, 9, aorta and its branches; 10, n, left auricle and ventricle. (Allen 

 Thomson.} 



FIG. 117. LEFT AURICLE AND VENTRICLE, OPENED AND PART OF THEIR WALLS REMOVED 

 TO SHOW THEIR CAVITIES, i, Pulmonary vein cut short; i', cavity of left auricle; 3,3", thick 

 wall of left ventricle; 4, portion of the same with papillary muscle attached; 5, the other papillary 

 muscles; 5', wall of the ventricle; 6, 6', the segments of the mitral valve; 7, the figure in aorta is 

 placed over the semilunar valves; 7', aorta; 8, pulmonary artery; 10, branches of aorta. (Allen 

 Thomson.) 



distention of the heart during contraction and to prevent separation of the 

 muscle-fibers. The endocardium is continuous with the lining membrane 

 of the blood-vessels. 



The inter- auricular septum is quite thin and composed of the two layers 

 of the endocardium, between which is a layer of muscle-fibers. It presents 

 at its lower portion an oval depression, the fossa ovalis. 



The inter- ventricular septum is quite thick and well developed, and com- 



