HEART 201 



are much more abundant than in the myocardium." The bundle con- 

 tains abundant ganglion cells and muscle libers. It is also very rich in 

 glycogen. 



It would seem, on the basis of its constancy of presence and structure, 

 and its probably independent blood and nerve supply, that the atrioventric- 

 ular bundle has a function independent of the myocardium, probably of 

 the nature of a neuromuscular end-organ (Retzer; DeWitt), providing for 

 the conduction of the impulse to contraction, and the coordination of the 

 atrial and ventricular rhythm. 



A muscle bundle of closely similar structure intimately related with 

 the vagus and sympathetic nerves, the ' sino-atrial node,' has been described 

 by Keith and Flack (1907) at the juncture of the sinus venosus and the 

 atrium. It is believed to be the place of origin of the impulse to the heart 

 beat, from which it is transmitted to the atrioventricular bundle. 



Laurens (Anat. Rec., 2, 8, 1913) has described an analogous muscular 

 connection between auricles and ventricles in certain reptiles, where it 

 assumes the form of an inverted funnel-shaped tube. 



Development of Heart. The anlage of the heart arises from the 

 fusion of a pair of parallel endothelial tubes in the paramedial angioblast, 

 each surrounded by primitive mesenchyma. The endothelium of the result- 

 ing sac differentiates into the endocardium of the definitive heart, while 

 the connective tissue and muscle develop from the mesenchyma in a man- 

 ner essentially similar to that described for the blood-vessels. 



Blood- Vessels. The heart is supplied with blood through the coro- 

 nary arteries. The larger branches of these vessels pursue their course 

 beneath the epicardium in the superficial grooves of the cardiac wall. 

 From these large arteries, smaller branches are distributed to the epi- 

 cardium and to the muscular wall, the latter vessels penetrating as far 

 as the endocardium, in whose connective tissue they form a meager 

 capillary plexus. 



The capillaries of the myocardium are extremely abundant. They 

 form elongated meshes between the muscle fibers, the circumference of 

 each muscle fiber being in relation with several capillary vessels. The 

 veins return the blood from these rich capillary plexuses and pursue a 

 course similar to that of the arteries, the larger veins being always found 

 in the broader connective tissue septa. In the right atrium certain small 

 veins, the vena 1 minima', empty directly into the cavity of the heart. 



The lymph supply is very abundant and intimate. The lymph ves- 

 sels form two superficial plexuses, the endocardial and the epicardial, 

 both draining into the larger lymph vessels at the base of the heart. 

 14 



