214 THE CIRCULATORY SYSTEM. 



separated from that of the ventricles by means of the annulus fibro- 

 sus atrioventricularis, or the auriculoventricular ring, which consists 

 in the adult of connective tissue containing numerous delicate and 

 densely interwoven elastic fibers. 



The pericardium consists of a visceral layer, the epicardium, ad- 

 hering closely to the myocardium, and a parietal layer (pericardium), 

 loosely surrounding the heart and continuous at the upper portion 

 of the heart with the visceral layer. Between the two layers is the 

 pericardial cavity, containing a small quantity of a serous fluid 

 the pericardial fluid. In the visceral layer (the epicardium) we 

 find a connective-tissue stroma covered by flattened mesothelial 

 cells. A similar structure occurs also in the parietal layer, although 

 here the connective -tissue stroma is considerably reinforced. De- 

 posits of fat, in most cases in the neighborhood of the blood-vessels, 

 are sometimes seen between the myocardium and the visceral layer 

 of the pericardium. 



According to Seipp, the distribution of the elastic tissue in the 

 heart is as follows : The endocardium of the ventricles contains far 

 more elastic tissue than that of the auricles, especially in the 

 left ventricle, where even fenestrated membranes may be present. 

 In the myocardium of the ventricles there are no elastic fibers aside 

 from those which are found in the adventitia of the contained blood- 

 vessels. In the myocardium of the auricles, on the contrary, such 

 fibers are very numerous and are continuous with the elastic 

 elements in the walls of the great veins. The epicardium also pre- 

 sents elastic fibers in the auricles continuous with those of the great 

 veins emptying into the heart, and in the ventricles continuous with 

 those in the adventitia of the conus arteriosus. In those portions 

 of the heart-wall containing no muscular tissue the elastic elements 

 of the epicardium are continuous with those of the endocardium. In 

 the new-born the cardiac valves possess no elastic fibers, although 

 they are present in the adult. They are developed on that side of 

 each valve, which, on closing, is the more stretched for instance, 

 on the auricular side of the auriculoventricular valves. 



The heart has a rich blood supply. The capillaries of the myo- 

 cardium are very numerous, and so closely placed around the 

 muscle bundles that each muscular fiber comes in contact with one 

 or more capillaries. In the endocardium the vessels are confined 

 to the connective tissue. The auriculoventricular valves con- 

 tain blood-vessels, in contradistinction to the semilunar valves, 

 which are non -vascular, while the chordae tendineae are at best very 

 poorly supplied with capillaries. 



The coronary arteries, which terminate in the capillaries above 

 mentioned, are terminal arteries in the sense that " the resistance in 

 the anastomosing branches is greater than the blood pressure in the 

 arteries leading to those branches (Pratt, 98). This observer has 

 further shown that the vessels of Thebesius (small veins which 

 open on the endocardial surfaces of the ventricles and auricles and 



