THE VASCULAR SYSTEM. 



communicate directly with all the chambers of the heart) "open 

 from the ventricles and auricles into a system of fine branches that 

 communicate with the coronary arteries and veins by means of 

 capillaries, and with the veins, but not with the arteries, by passages 

 of somewhat larger size"; so that, although the blood supply through 

 the coronary arteries for a given area of the myocardium is cut off, 

 the heart muscle of this area may receive blood through the vessels 

 of Thebesius. 



Lymphatic netivorks have been shown to exist in the endocar- 

 dium, and their presence in the pericardium is not difficult to demon- 

 strate. Little is known with regard to the lymph -channels of the 

 myocardium. 



The nerve supply of the heart includes numerous medullated 

 nerve-fibers, the dendrites of sensory neurones, and numerous non- 

 medullated fibers, the neuraxes of sympathetic neurones. Smirnow 

 (95) described sensory nerve -endings in the endocardium of amphibia 

 and mammalia, which he suggests may be the terminations of the 

 depressor nerve. Dogiel (98) has corroborated and extended these 

 observations, and has described complicated sensoiy telodendria 

 situated both in the endo- and pericardium. The latter states that, 

 after forming plexuses and undergoing repeated division, the medul- 

 lated sensory nerves lose their medullary sheaths, the neuraxes 

 further dividing in numerous varicose fibers, variously interwoven 

 and terminating in telodendria, which vary greatly in shape and 

 configuration. These tek>dendria are surrounded by a granular 

 substance containing branched cells, probably connective-tissue 

 cells, the interlacing branches of which form a framework for the 

 telodendria. Similar sensory nerve-endings occur in the adventitia 

 of the arteries and veins of the pericardium (Dogiel, 98) ; and 

 Schemetkin has shown that sensory nerve-endings occur in the adven- 

 titia and intima, especially in the latter, of the arch of the aorta and 

 pulmonary arteries. In the heart, under the pericardium on the 

 posterior wall of the auricles and in the sulcus coronarius, are found 

 numerous sympathetic neurones whose cell-bodies are grouped 

 to" form sympathetic ganglia. The neuraxes of these sympathetic 

 neurones varicose, nonmedullated nerve-fibers form intricate 

 plexuses situated under the pericardium and, penetrating the myo- 

 cardium, surround the bundles of heart muscle-fibers. From the 

 varicose nerve-fibers constituting these plexuses, fine branches are 

 given off, which terminate on the heart muscle-cells in a manner 

 previously described (see p. 166 and Fig. 132). The cell-bodies of 

 the sympathetic neurones, the neuraxes of which thus terminate 

 on the heart muscle-fibers, are surrounded by end-baskets, the 

 telodendria of small medullated nerve-fibers which reach the heart 

 through the vagi. The slowed and otherwise altered action of the 

 heart-muscle, produced on stimulating directly or indirectly the 

 vagus nerves is therefore due not to a direct action of these nerve- 

 fibers on the heart muscle-cells, but to an altered functional activity 



