THE rOSTERIOIi AuETA. 609 



Cardiac or Coronary Arteries (Figs. 349, 350). 



There are two cardiac arteries — a right and left — exclusively destined for the 



tissue of the heart. 



Right Coronary Artery (Figs. 349, 1 ; 350, /)• — 'I'tiis originates from 

 the front and to the right of the aorta, at the free margin of the semilunar 

 valves, and proceeds perpendicularly, or at a rigiit angle, from the trunk, passing 

 forwards to the right of the pulmonary artery, beneath the anterior auricle ; 

 then to the right aud backwards, to reach the auriculo-ventriculur groove, which 

 it follows till near the origin of the right ventricular groove. Here it divides 

 into two branches — one vertical, descending in this groove to the apex of the 

 heart, which it bends round to the front, and anastomoses with an analogous 

 branch of the left coronary artery ; the other is horizontal, smaller than the 

 first, and follows the original course of the artery in the auriculo-ventricular 

 groove, also inosculating with the artery of the left side. 



Left Coronary Artery (Fig. 349, 2). — This arises opposite the preceding, 

 at the same angle of incidence, passes behind the pulmonary artery, and divides — 

 under the left or posterior auricle — into two branches similar in every respect to 

 those of the right artery. The vertical branch descends in the left perpendicular 

 groove ; the horizontal is lodged in the coronary groove ; and both anastomose 

 with the analogous branches of the opposite vessels. 



From this arrangement, it results that the heart is surrounded by two arterial 

 circles — a vertical, or ventricular, which has been compared to a meridian ; and 

 a horizontal, or auriculo-ventricular, analogous to an equatorial circle. 



In their course — which is more or less tortuous — the coronary arteries throw 

 out a considerable number of ramuscules, which enter the muscular tissue of the 

 heart. The vertical circle gives off branches that are entirely ventricular ; while 

 from the horizontal circle come the superior or auricular, and inferior or ventri- 

 cular branches. Among the latter there is one which, rising from the right 

 artery — where it bends at an angle beneath the auricle — enters the substance of 

 the right ventricle by passing round the pulmonary infundibulmn ; its ramifica- 

 tions anastomose with those of a similar branch from the left artery, and in this 

 way establishes another communication between the two vessels. 



Aeticle II. — Posterior Aorta. 



Course. — This artery (Fig. 349, 9) is a continuation of the aorta, which it 

 nearly equals in volume, and from which it passes upwards and backwards, de- 

 scribing a curve the convexity of which is antero-superior, and which is known 

 as the arch of the aorta. It thus reaches the left side of the inferior face of the 

 spine, about the seventh dorsal vertebra, behind the posterior extremity of the 

 longus colli muscle, and is then carried directly backwards, following the bodies 

 of the vertebrae, though a little to the left at first ; it gradually inclines to the 

 right, however, and reaches the median plane at the pillars of the diaphragm. 

 Here it passes through the opening circtimsci'ibed by these two pillars {hiatus 

 aorticus), enters the abdominal cavity, and extends to the entrance of the pelvis, 

 under the spine, still preserving its miedian position. On reaching the last 

 intervertebral articulation, the posterior aorta terminates by a double bifurcation, 

 from which arise the external and internal iliac arteries. 



