HEART. 



135 



and in looser form it unites with the mesothelium to make the epicardium. 

 The epicardium or -visceral pericardium is continuous with the parietal 

 pericardium in such a way that the two layers form a closed sac which 

 envelops all of the heart except its base, where the large vessels enter and 

 leave it. The pericardial cavity within this sac was originally continuous 

 with the peritoneal cavity, and in the adult the walls of these subdivisions 

 of the coelom have essentially the same structure. It contains the serous 

 pericardial fluid. 



Adult structure of the heart. 

 The endocardium is a connective 

 tissue layer covered with an en- 

 dothelium composed of irregu- 

 larly polygonal cells. It contains 

 some smooth muscle fibers, and 

 elastic networks which, in the 

 atria especially, form fenestrated 

 membranes. In the deeper part 

 of the endocardium, partially 

 developed cardiac muscle fibers 

 occur in some mammals, but 

 rarely in the human adult. Such 

 muscle fibers, characterized by 

 containing only a peripheral ring 

 of banded fibrils, are called 

 Turkinje's fibers.' They may 

 be transformed into typical car- 

 diac muscle. The valves of 

 the heart are essentially folds 

 of endocardium containing 

 dense fibro-elastic tissue con- 

 tinuous with the annuli fibrosi. 



In the atrioventricular valves there are smooth muscle fibers, most abundant 

 near the attacked borders; and some blood vessels. The semilunar valves 

 of the pulmonary artery and aorta consist of connective tissue which is 

 denser and more elastic on the side toward the ventricles, and particularly 

 at the periphery and nodules of the valves. The nodules are thickenings 

 in the center of the circumference of each segment of the valve, which 

 perfect their approximation when closed. The endocardium contains 

 free sensory nerve endings, associated with modified connective tissue cells, 

 and undoubtedly motor nerves to its few muscle fibers. Lymphatic 

 vessels have been described in it, together with the terminal capillaries 



FIG. 160. SECTION OP THE HEART SHOWN IN FIG. 72, 

 F. 



ca., Capillaries; en., endothelium; L a., left atrium; 

 1. v., left ventricle; mes., mesothelium (of the 

 epicardium, or visceral pericardium) ; p. c., peri- 

 cardial cavity; p. 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. 



