196 THE BLOOD VASCULAR SYSTEM 



small openings at the angles between its cells; these stomata are sur- 

 rounded by minute, finely granular cells and are perhaps connected with 

 the lymphatic vessels. 



The mesothelium of the epicardium is supported upon a thin layer 

 of dense areolar tissue in which are many small blood-vessels and 

 lymphatics. Fibers from the deeper surface of this layer are prolonged 

 into the myocardium to become continuous with its endomysial connec- 

 tive tissue. The larger of these connective tissue trabeculas accompany 

 the branches of the larger arteries and veins which are distributed to 

 the muscular wall of the heart. 



Endocardium. The endocardium consists of a lining membrane of 

 polygonal cndothelial cells supported upon a thin layer of delicate 



fibrous connective tissue, of en- 

 dothelial origin (Mall). In this 

 membrane is a network of elas- 

 tic fibers, and a small amount 

 of smooth muscle. The en- 

 dothelium of this membrane is 

 continuous with that of those 

 blood-vessels which open from 



212. THE PARIETAL LATER OF THE the cav i t i es of the heart. Its 

 PERICARDIUM OF A CHILD. ,. , . IP 



connective tissue also forms a 



a, mesothelium ; o, connective tissue. t . . . . , 



Hematein and eosin. Photo. X 500. continuous layer with that of 



the tunica intima of these ves- 

 sels; in fact, the three coats of the cardiac wall endocardium, myocar- 

 dium, and epicardium might well be compared with the corresponding 

 three coats of the arterial and venous walls the intima, media, and ad- 

 ventitia. In either organ, the inner coat consists of a lining membrane 

 of endothelium, and a supporting membrane of connective tissue; muscle 

 in large part composes the middle coat, while the outer coat is typically a 

 connective tissue layer. 



Valves. At the cardiac orifices the entire thickness of the endocardium 

 is folded upon itself to form a double layer, between the folds of which 

 an intervening stratum of dense fibro-elastic tissue is inserted. These 

 endocardial folds form the cardiac valves. The number and shape of 

 their cusps are dependent upon the location. The semilunar valves of 

 the aortic and pulmonary orifices consist of three crescentic endocardial 

 folds; at the auriculoventricular orifices the tricuspid valve consists of 

 three, the bicuspid or mitral of two, folds. 



The margin of the valvular cusp or fold is extremely thin; just within 



