428 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



one of which descends along the anterior interventricular groove to 

 the apex of the heart, while the other follows the left auriculoventricu- 

 lar groove. From these superficial blood-vessels, forming what is 

 known as the extramural system, branches are given off which pene- 

 trate the substance of the heart and by repeated division give rise to 

 the so-called intramural system. 



The cardiac veins follow in the course of the arteries, uniting 

 eventually in the coronary sinus which is about one inch in length 

 and occupies the inferior extremity of the left auriculoventricular 

 groove. It empties into the right auricle in front of the inferior caval 

 opening, its orifice being guarded by the valve of Thebesius. 



In this connection it should be recalled that the hearts of those 

 iower forms, which are not in possession of an independent circulation, 

 obtain their nutritive material directly from the blood as it traverses 

 the cardiac chambers. Many of these organs also contain irregular 

 tubular passages which penetrate the musculature and thus enable 

 the blood to come into contact with even the most remote cells. A 

 similar arrangement is present in the mammalian heart. Numerous 

 openings, the so-called foramina of Thebesius, establish a communica- 

 tion with a system of tubules which ramify below the endocardial 

 membrane, 1 but the nourishment which the mammalian heart is able to 

 derive from this source is not sufficient for its metabolic requirements. 2 



That the activity of the mammalian heart is actually dependent 

 upon the coronary blood supply, may readily be gathered from the 

 fact that an isolated and quiescent organ may be made to beat again 

 by instituting an artificial circulation through its coronary circuit. 

 In fact, the frequency and force of the cardiac contractions invariably 

 go hand in hand with the pressure under which the perfusion is made. 

 Very similar results may be obtained at times with the heart of the cat. 

 Thus, if its aortic orifice is occluded, it ceases to beat almost imme- 

 diately, but may be made to contract again by filling its chambers 

 with defibrinated blood under a pressure of about 75 mm. Hg. This 

 procedure, however, is not so reliable as the perfusion through the 

 coronary artery. This fact, that it resumes its activity under these 

 circumstances, might also be explained upon the basis that the -cat's 

 heart possesses a more extensive system of direct nutritive channels 

 than that of other mammals. The ligation of the coronary arteries 

 in the dog is followed almost immediately by a diminution in the 

 rate and force of the heart beat and eventually by a complete stoppage. 

 In fact, Parker has shown that the occlusion of one of its branches, 

 namely the circumflex artery, suffices to arrest the heart in about 

 80 per cent, of the animals. 



While the superficial cardiac vessels are protected in a measure 

 by the visceral layer of the pericardium, as well as by connective 

 tissue and fat, the deeper branches are directly exposed to the power 



1 Pratt, Am. Jour, of Physiol., i, 1898, 86. 



2 Langendorff, Pfltiger's Archiv, Ixi, 1895, 291. 



