284 TEXT-BOOK OF PHYSIOLOGY. 



over the walls of both auricles and ventricles, the most important 

 of which in man are the anterior and posterior interventricular. 

 These main vessels lie in grooves on the surface of the heart beneath 

 the visceral pericardium, surrounded by connective tissue and fat. 

 Small branches penetrate the heart-muscle in which they break up 

 into capillaries. From the capillary areas small veins arise which, 

 passing backward, converge to form the coronary veins. These 

 follow the course of the arteries an\finally terminate in the coronary 

 sinus, located in the auriculo-ventricular groove on the posterior 

 surface of the heart. t\ 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 been lately 

 shown by PratUhat these foramina are present not only in the auricular 

 wall, as generally stated, but in the walls of all the cavities. These 

 foramina communicate through a capillary plexus with both arteries 

 and veins, and by special large passages with the veins alone. 



During the systole the intra-mural vessels are compressed and the 

 blood driven out of the capillaries into the veins; during the diastole, 

 the vessels again dilate and permit the blood to re-enter freely from 

 the arteries. The greater the force and_frec[uency_of the beat, the 

 greater the volume of blood passing through the coronary system. 



The period of time in the cardiac cycle during which the coronary 

 arteries are filled with blood, whether during the systole or the dias- 

 tole, has been a subject of much discussion.\ V At present, however, 

 as the result of many experiments it is generally believed that they 

 are filled at the time of the systole.\ V A comparison of the tracings of 

 the pulse- wave taken simultaneously in the carotid and coronary 

 arteries shows that the pressure rises and falls simultaneously in both 

 vessels; that there is a complete agreement between the two trac- 

 ings, and as a corollary both vessels are filled during the systole. 

 But because of the pressure which the heart muscle must exert upon 

 the smaller arteries and veins within its own substance during systole, 

 it is probable that there is a freer circulation in the coronary vessels 

 during the period of diastolic repose. 



In mammals the nutrition of the heart-muscle, its irritability and 

 contractility, depend on the blood-supply derived from the coronarv 

 vessels. This is shown by the effects which follow its withdrawal. 

 Ligation of both coronary arteries in the dog is followed by a diminu- 

 lon in the force and frequency of the heart-beat, and in a few minutes 

 by complete cessation. Ligation of even a single branch of a coro- 



