CIRCULATION. 471 



observed that after the ligature just described had been drawn tight, thus 

 arresting the heart, the placing of a second ligature around the heart at the 

 junction of the auricle and ventricle caused the latter to begin to beat again, 

 while the auricle remained at rest. This second ligature, it is generally 

 admitted, stimulates the ganglion of Bidder, and the ventricle responds by 

 rhythmic contractions to the constant excitation thus produced. Loosening the 

 ligature and so interrupting the excitation stops the ventricular beat. 1 



PART III. THE NUTRITION OF THE HEART. 



The cells of which the heart-wall are composed are nourished by contact 

 with a nutrient fluid. In hearts consisting of relatively few cells no special 

 means of bringing the nutrient fluid to the cells is required. The walls of the 

 minute globular heart of the small crustacean Daphnia, for example, are com- 

 posed of a single layer of cells, each of which is bathed by the fluid which the 

 heart pumps. In larger hearts with thicker walls only the innermost cells 

 could be fed in this way. Special means of distributing the blood throughout 

 the substance of the organ are necessary here. 



Passages in the Frog's Heart. In the frog this distribution is accom- 

 plished chiefly through the irregular passages which go out from the cavities 

 of the heart between the muscle-bundles to within even the fraction of a milli- 

 meter of the external surface. 2 These passages vary greatly in size. Many are 

 mere capillaries. They are lined by a prolongation of the endothelium of the 

 heart. Filled by every diastole and emptied by every systole, they do the 

 work of blood-vessels and carry the blood to every part of the cardiac muscle. 

 Henri Martin 3 describes a coronary artery in the frog, analogous to the 

 coronary arteries of higher vertebrates. This artery supplies a part of the 

 auricles and the upper fourth of the ventricle. 



In the rabbit, cat and dog, and in man a well-developed system of cardiac 

 vessels exists, the coronary arteries and veins. Their distribution in the dog 

 deserves especial notice, because the physiological problems connected with these 

 vessels have been studied chiefly in this animal. 



Coronary Arteries in the Dog. In the dog the coronary arteries and 

 their larger branches lie upon the surface of the heart, covered as a rule only 

 by the pericardium and a varying quantity of connective tissue and fat. The 

 left coronary artery is extraordinarily short. A few millimeters after its origin 

 from the aorta it divides into the large ramus circumflex and the descen- 

 dens, nearly as large. The former runs in the auriculo-ventricular furrow 

 around the left side of the heart to the posterior surface, ending in the pos- 

 terior inter- ventricular furrow. The left auricle and the upper anterior and the 

 posterior portion of the left ventricle are supplied by this artery. The descen- 

 dens runs downward in the anterior inter- ventricular furrow to the apex. Close 

 to its origin the descendens gives off the arteria septi, which at once enters the 



1 Goltz, 1861, p. 201. 2 Engelmann, 1874, p. 11. 8 Martin, 1893, p. 754 ; 1894, p. 46. 



