166 THE ORGANS 



manner, becoming constantly deeper, to terminate in the papillary 

 muscles of the opposite ventricle. The deeper fibres pass from the 

 auriculo-ventricular ring around the ventricle of the same side, 

 through the interventricular septum and terminate in the papillary 

 muscles of the opposite ventricle. 



The ENDOCARDIUM covers the inner surface of the myocardium 

 and forms the serous lining of all the chambers of the heart. At 

 the arterial and venous orifices it is seen to be continuous with and 

 similar in structure to the intima of the vessels. It consists of two 

 layers: (a) an inner composed of a single layer of endothelial cells, 

 corresponding to the endotheHal Hning of the blood-vessels; and (b) 

 an outer composed of mixed fibrous and elastic tissue and smooth 

 muscle cells. Externally the endocardium is closely attached to the 

 myocardium. 



Strong fibrous rings {annuli fibrosi), composed of mixed fibrous 

 and elastic tissue, surround the openings between auricles and 

 ventricles. Similar but more delicate rings encircle the openings 

 from the heart into the blood-vessels. 



The heart valves are attached at their bases to the annuli fibrosi. 

 They are folds of the endocardium, and like the latter consist of 

 fibrous and elastic tissue continuous with that of the rings and 

 covered by a layer of endothelium. 



The EPiCARDiUM is the visceral layer of the pericardium. It is 

 a serous membrane like the endocardium, which it resembles in 

 structure. It consists of a layer of mixed fibrous and elastic tissue 

 covered over by a single layer of mesothelial cells. Beneath the 

 epicardium there is usually more or less fat. 



Blood-vessels.^ — Blood for the nutrition of the heart is supplied 

 through the coronary arteries. The larger branches run in the con- 

 nective tissue which separates the bundles of muscle fibres. From 

 these, smaller branches pass in among the individual fibres, where 

 they break up into a rich capillary network with elongated meshes. 

 From the myocardium, capillaries penetrate the connective tissue 

 of the epicardium and endocardium. The auriculo-ventricular 

 valves are supplied with blood-vessels, while in the semilunar valves 

 blood-vessels are wanting. 



Lymphatics. — ^Lymph channels traverse the epicardium and 

 endocardium and enter the valves. Within the myocardium 

 minute lymph vessels have been demonstrated between the muscle 

 fibres and accompanying the blood-vessels. 





