THE CIRCULATION OF THE BLOOD 295 



hours, thus showing that the mammalian heart may to some extent so 

 receive nutritive material. By reason of the fact that the metabolism of the 

 heart of the mammal is so much more active than that of the heart of the 

 frog, this method is far from being sufficient for nutritive purposes and hence 

 a more perfect and active blood-supply is necessitated for furnishing 

 nutritive material and the removal of the waste products. These results are 

 accomplished by the coronary arteries, on the one hand, and the coronary 

 veins, on the other. 



The Coronary Vessels. The coronary arteries, two in number, the 

 right and left, arise from the aorta in the pouches .of Valsalva just above 

 the right and left semilunar valves. Turning in opposite directions, they 

 form a circle around the base of the ventricles and ultimately anastomose. 

 From both the right and left artery branches are given off which run over 

 the walls of both auricles and ventricles, the most important of which in 

 man are the anterior and posterior inter-ventricular. These main vessels 

 lie in grooves on the surface of the heart beneath the visceral pericardium, 

 surrounded by connective tissue and fat. From their relation to the outer 

 surface of the heart they may be designated extra-mural vessels. From these 

 vessels small branches are given off \frhich penetrate the walls of the heart, 

 in which they divide into many branches and because of their relation to 

 the heart-muscle they may be designated intra-mural vessels. These vessels 

 are spoken of as terminal arteries in the sense that "the resistance in the 

 anastomosing branches is greater than the blood pressure in the- arteries 

 leading to those branches" (Pratt). From these small arteries arise a 

 rich network of capillary blood-vessels which closely invest the individual 

 muscle cells, and which permit here a rapid and extensive interchange of 

 nutritive and waste materials. The coronary veins arise from the union 

 of the small veins which emerge from the capillary areas. The veins follow 

 the course of the arteries and finally terminate in the coronary sinus, located 

 in the auriculo-ventricular groove on the posterior surface of the heart. 

 This sinus opens into the right auricle between the opening of the inferior 

 vena cava and the auriculo-ventricular opening. Its orifice is guarded by 

 a valve, which is usually single, though sometimes double. 



While by far the larger portion of the blood is returned by the coronary 

 veins, it is also certain that some of it is returned by small veins which open 

 into little pits or depressions on the inner surface of the heart-walls, known 

 as the foramina Thebesii. It has, however, been shown by Pratt that these 

 foramina are present not only in the auricular walls, as generally stated, but 

 in the walls of the ventricular cavities as well. They communicate through 

 a capillary plexus with both arteries and veins, and by special large passages 

 with the veins alone. 



The Filling of the Coronary Arteries. The period of time in the 

 cardiac cycle during which the coronary (the extra-mural) arteries are filled 

 with blood, whether during the systole or the diastole, has been a subject of 

 much discussion. Thus it was asserted and maintained by Briicke that 

 this event must occur during the diastole, because of the supposed fact that 

 the semilunar valves during the systole are so closely pressed against the 

 walls of the aorta and over the openings of the coronaty arteries as to prevent 

 the entrance of blood into them; but with the diastole and the return of the 

 valves to their former position the blood under the recoil of the walls of the 



