HEART l8l 



rich in glycogen. In the node, however, according to Miss DeWitt, the 

 fibers, though varying greatly in size, are much smaller than those found 

 elsewhere in the heart. Several of them unite at a point, producing stel- 

 late groups, and the entire node is an intricate network. 



The fibers of the atrio-ventricular bundle resemble those described by Purkinje 

 in the sheep, horse, cow and pig, but which he could not find in the rabbit, dog and man 

 (Arch. f. Anat., Physiol. u. wiss. Med., 1845, pp. 281-295). I n the walls of the ven- 

 tricle, immediately beneath the endocardium, he observed "first with the naked eye, a 

 network of gray, flat gelatinous threads, which in part were prolonged into the papillary 

 muscles, and in part passed like bridges across the separate folds and clefts." Under 

 the microscope, they appeared very granular, but he decided that they were probably 

 muscular. Purkinje's fibers are regarded as imperfectly developed muscle fibers. 

 In the human heart they are not as distinct from the other cardiac muscle fibers as in 

 the sheep. It is possible that they are directly continuous with the cardiac syncy- 

 tium, although, as noted by Miss DeWitt, if the transition is gradual it will be very 

 difficult to observe in sections. 



At the junction of the superior vena cava and the atrium, Keith and 

 Flack have described a peculiar musculature imbedded in densely packed 

 connective tissue, composed of striated, fusiform fibers, plexiform in ar- 

 rangement, with .well-marked elongated nuclei, "in fact, of closely similar 

 structure to the node" (Journ. Anat. and Physiol., 1907, vol. 41, pp. 

 172-189). These fibers are said to be in close relation with the vagus and 

 sympathetic nerves; they have a special arterial supply. According to 

 Keith and Flack they are situated at the junction of the sinus venosus 

 and the atrium, and they form the sino-atrial node (sino-auricular node). 

 The sino-atrial node is found immediately beneath the epicardium in the 

 position shown in Fig. 173. In it the impulse for the heart beat is be- 

 lieved to originate, and to be transmitted to the atrio-ventricular node; 

 the latter correlates the contraction of the atrium with that of the 

 ventricle. 



Epicardium. The epicardium is a connective tissue layer, covered 

 with simple flat mesothelium and containing elastic fibers and many fat 

 cells. The latter are distributed along the course of the blood vessels. 



Vessels and Nerves. The branches of the coronary vessels pass from 

 the epicardium into the myocardium, forming capillaries in intimate 

 relation with the muscle fibers. The heart is thus supplied with aerated 

 blood from the root of the aorta, as well as by the blood within its own 

 cavities; on the left side this is aerated, but not on the right. 



The lymphatic vessels, draining toward the base of the heart, are 

 very abundant, and true lymphatic vessels are found in all layers of the 

 heart. The tissue spaces in the myocardium are also extensive. 



The nerves to the heart have already been described as forming the 

 cardiac plexus. This plexus receives branches from the vagus, and from 

 the sympathetic cardiac nerves proceeding from the cervical sympathetic 



