704 HUMAN ANATOMY. 



supplied by the right one, or vice versa, and sudden occlusion of either of the arteries 

 will produce serious disturbances or, in some cases, complete arrest of the contrac- 

 tions of the heart. Since, however, the capillaries of the heart's substance, into 

 which each artery is continued, form a continuous net-work, a passage-way for the 

 blood of one artery into the territory normally supplied by the other may be formed 

 by their enlargement, opportunity for which may be afforded in cases in which the 

 occlusion of an artery has been very gradual in its development. 



There is, however, another way by which the tissue of the heart may receive nutrition in 

 cases of gradual occlusion of the coronary arteries, namely, through the Thebesian orifices in 

 the walls of the auricles and ventricles. These openings communicate by means of capillaries 

 with the coronary vessels, and it has been shown experimentally that the heart can be effectively 

 nourished by blood passing from the chambers of the heart through the Thebesian vessels and 

 back into the coronary veins. 



The lymphatic vessels of the heart form a net-work beneath the visceral layer 

 of the pericardium, and a second less distinct net-work has also been described as 

 occurring beneath the endocardium. These net-works communicate with two sets of 

 principal vessels which lie in the anterior and posterior portions of the auriculo-ven- 

 tricular groove. The anterior set passes from the right to the left, and, on reaching 

 the pulmonary aorta, passes around its left surface to reach the systemic aorta, upon 

 which it ascends to terminate in a lymphatic node situated to the left of the trachea. 

 The posterior set opens in part into the anterior one and in part ascends along the 

 right side of the pulmonary aorta to terminate in one of the nodes situated beneath 

 the bifurcation of the trachea. 



The Nerves of the Heart. — The nerves of the heart are derived from the 

 cardiac plexuses and, passing downward along the aortae, are distributed partly to the 

 auricles and partly accompany the coronary arteries along the auriculo-ventricular 

 groove, where they form the coronary plexits, from which branches are distributed 

 to the ventricles. Over the surfaces of the auricles and ventricles the branches form 

 a fine plexus situated beneath the visceral layer of the pericardium, and from this 

 plexus branches pass into the substance of the heart to terminate upon the muscle- 

 fibres. Some nerve-fibres are also distributed to the pericardium and endocardium, 

 and these are regarded as being afferent in function, as are also certain fibres which 

 terminate in the connective tissue of the heart's walls. 



Scattered in the superficial plexuses there are numerous ganglion-cells, some- 

 times occurring singly and sometimes collected into small ganglia. They tend to be 

 especially numerous around the orifices of the great veins opening into the auricles, 

 in the coronary plexuses, and over the upper portions of the ventricles. It has been 

 asserted that ganglion-cells also occur embedded in the walls of the \'entricles, but at 

 present this requires confirmation. 



Much has yet to be learned concerning the qualities of the various nerve-fibres which pass to 

 the heart in man, but it is presumable that they resemble in general those which have been deter- 

 mined experimentally for those of the lower mammals, bi the latter it has been shown that the 

 cardiac plexuses contain both afferent and efferent nerve-fibres. The cardiac plexuses are formed 

 by branches from the pneumogastric and sympathetic nerves, and among the fibres from the former 

 nerve are some which, when stimulated, cause a cessation of the heart's contractions, whence 

 they are termed the itthibitory nerves. Stimulation of sympathetic fibres, which, in the dog, for 

 example, pass to the cardiac plexus from the first thoracic ganglion of the ganglionated cord, 

 produces an increase in either the rapidity or the intensity of the heart-beat, and these fibres are 

 consequently termed the accelerator or augmentor fibres. Both the inhibitor)' and augmentor 

 fibres are efferent, — i.e., they carry impulses from the nerve-centres out to the heart ; in addi- 

 tion, the existence of afferent fibres has been determined among the pneumogastric constituents 

 of the plexuses. These are the depressor fibres, so called because their stimulation produces a 

 marked fall in the blood-pressure, not on account of any action upon the heart-beat, since they 

 lead the stimulus away from the heart, but by acting reflexly upon the intestinal \essels so as to 

 produce their dilatation and, by thus lessening the peripheral resistance against which the heart 

 must contend, lessen the work which the organ has to do. 



Whether the various efferent fibres pass directly to the muscular tissue of the heart or ter- 

 minate upon cardiac ganglion-cells which transmit the impulse to the muscle-fibres is a point 

 which remains to be determined, although, from analogy with what is known as to the relation 

 of the cerebro-spinal fibres to other portions of the involuntary muscular tissue, it would seem 

 probable that the pneumogastric efferent fibres terminate primarily upon the cardiac ganglion- 

 cells. 



