THE HEART. 103 



into a right and left limb, the bundle continues into the interventricular 

 partition. Although distinct and compact during this course, at its two 

 ends the atrio-ventricular bundle breaks up into radiating and interlacing 

 strands, which form an intricate network composed of Purkinje fibres. The 

 latter disappear among the elements of the myocardium by gradual transition 

 into the ordinary fibres of cardiac muscle. The Purkinje fibres ramify not 

 only within the septum between the heart-chambers, but also invade the tra- 

 becula (columnae carneae) and papillary muscles of the ventricles. For a 

 time muscular throughout their length, the papillary muscles become trans- 

 formed into the fibrous chordae tendineae in the segments attached to the 

 valves. These fibrous cords often contain considerable elastic tissue which 

 is continuous with the fibre-elastic layer of the valve-leaflets. 



The Epicardium. The external layer of the heart-wall, the epicar- 

 dium, corresponds in its general structure with other parts of the pericar- 

 dium. It consists, as do other serous membranes, of a single layer of endo- 

 thelial cells that covers the free surface of the heart and rests upon a stratum 

 of fibre-elastic connective tissue. The elastic fibres are very fine and 

 numerous and form a dense network immediately beneath the endothelium. 

 Those within the atrial epicardium are prolonged into the adventitia of 

 the great veins, while the elastic fibres of the ventricular covering end before 

 reaching the aorta and pulmonary artery. Where not separated from the 

 muscle by subserous fat, which may be abundantly present even in nor- 

 mal hearts, especially in the atrio-ventricular and the interventricular 

 grooves, the epicardium is intimately attached to the subjacent muscular 

 coat. The numerous branches of the coronary vessels, as well as the nerve- 

 trunks and the microscopic ganglia connected with the coronary plexuses, 

 lie beneath the epicardium or within its deepest layer. 



Blood-Vessels. The unusually generous vascular supply of the heart 

 includes the branches derived from the coronary arteries and the capillaries. 

 The former ramify beneath the epicar-dium and are, to some extent, end- 

 arteries, that is, arteries which do not directly anastomose with their neighbors. 

 Although both the epicardium and the deepest layer of the endocardium 

 contain small vessels destined for their tissues, it is to the heart-muscle that 

 the blood is chiefly directed. The larger vessels of the myocardium course 

 within the more robust tracts of connective tissue, giving off the twigs which 

 resolve into the capillary networks. These exhibit elongated meshes, similar 

 to those seen in voluntary muscle, which enclose the muscle-fibres. The re- 

 lation of the capillaries to the individual fibres is most intimate, since in many 

 places the capillaries are received in grooves, or almost tunnels, in the muscle- 

 substance. . The valves are devoid of blood-vessels, with the exception of 

 those accompanying muscular tissue within the bases of the auriculo-ventric- 

 ular leaflets. 



The lymphatics of the heart are represented by the numerous lymph- 

 spaces within the connective tissue between the muscle-bundles, and by the 

 more definite lymphatic vessels. The latter form two sets, a network within 

 the deepest layer of the endocardium and a network beneath or within the 

 epicardium. These networks communicate with the larger lymphatic vessels 

 which lie in the atrio-ventricular groove. 



The nerves are many and contributed by the vagus and the sympathetic. 

 They include both medullated and nonmedullated fibres which form the cor- 

 onary and many small subsidiary plexuses. Scattered in these superficial 

 plexuses lie numerous nerve-cells, sometimes singly but often collected into 

 microscopic ganglia. They are especially plentiful around the orifices of the 



