THE CIRCULATION OF THE BLOOD. 287 



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 arteries, two in number, the right and left, arise 

 from the aorta in the pouches of Valsalva just above the right and left semi- 

 lunar valves. Turning in opposite directions, they ultimately anastomose, 

 forming a circle around the base of the ventricles. 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 which penetrate the walls of the heart, in which they divide 

 into many branches before terminating in a capillary system. Because of 

 their relation to the heart-muscle they may be designated intra-mural vessels. 

 From the capillary areas small veins arise which, passing backward, con- 

 verge to form the coronary veins. These follow the course of the arteries 

 and finally terminate in the coronary sinus, located in the auriculo-ventricu- 

 lar 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 coronary 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 flows freely into them. It was 

 further assumed that the coronary arteries empty themselves into the capil- 

 laries during the time of the systole. According to Briicke the emptying of 

 the coronary arteries and the consequent fall of pressure within them pro- 

 moted the contraction of the ventricle, while the filling of the vessels and the 

 consequent rise of pressure facilitated the diastole. This anatomic mechan- 

 ism and its associated functional activity constituted according to Briicke an 

 apparatus by which the activity of the heart could be self-regulated. This 

 theory, however, has been disproved and is no longer entertained. 



