HISTOLOGY 



in the adult is divided into three layers, the endocardium, myocardium 

 and epicardium respectively. The endocardium consists of the endothe- 

 lium, which is continuous with that of the blood vessels, and of subendo- 

 thelial fibrous tissue. According to Mall, this tissue is derived from the 

 endothelium. The myocardium is the muscle layer, which is thin in the 

 atria, but very thick in the ventricles; in the left ventricle it is much 

 thicker than in the right. The epicardium consists of the pericardial epi- 

 thelium together with underlying connective tissue. This layer is also 

 called the visceral pericardium, and with the parietal pericardium it bounds 

 the pericardial cavity, forming a closed sac containing the pericardial 



fluid. The general relations of 

 these layers in an embryonic 

 heart are shown in Fig. 171. 

 The epicardium is a smooth 

 layer. The musculature of the 

 ventricles is arranged in trabec- 

 ulae covered with endothelium, 

 between which there are blood 

 spaces classed as sinusoids. In 

 the adult the musculature is 

 more compact, but internally it 

 is indented by many clefts and 

 irregular spaces, extending 

 among the trabecula carnea and 

 the conical papillary muscles. 



Endocardium. The endo- 

 cardium consists of endothelium 

 which is a single layer of flat, 

 irregularly polygonal cells, and 

 of the underlying connective 

 tissue which contains smooth 

 muscle and many elastic fibers 



FIG. 171. SECTION OF THE HEART SHOWN IN FIG. 170. F. 



ca., Capillaries; en., endothelium; 1. a., left atrium; 1. v., 

 left ventricle; mes., mesothehum (of the epicardium. 

 or visceral pericardium) ; p. c., pericardial cavity; j>. p., 

 parietal pericardium; r. a., right atrium; r. v., right 

 ventricle; si., sinusoids; v. b., bicuspid valve; v. t., 

 tricuspid valve; v. v. s., valves of the venous sinus 



(Fig. 172). Elastic fibers are more highly developed in the atria than in 

 the ventricles; they occur either as networks of thick fibers or fuse to 

 form fenestrated membranes. Smooth muscle fibers are more numerous 

 where the wall of the heart is smooth; they are most abundant in 

 front of the root of the aorta. 



The a trio- ventricular valves are essentially folds of endocardium 

 containing dense fibro-elastic tissue continuous with the similar tissue 

 in the annuli fibrosi. The valves contain muscle fibers toward these 

 rings, and elastic fibers which are prolonged into the chorda tendinece. 

 Blood vessels are found only in the basal portion of the valves, where the 

 muscle fibers occur. The semilunar valves of the pulmonary artery and 



