THE POSTERIOR AORTA. 523 



Included, on the rigbt side, in the crescent formed by the auricular 

 mass ; in relation, on the left side, with the pulmonary artery, which 

 is joined to it by means of cellulo-adipose tissue traversed by the 

 cardiac nerves, the common aorta forms, with the latter artery, a 

 fasciculus enveloped by the visceral layer of the pericardium, which is 

 reflected as a sheath around these two vessels. 



Two collateral arteries are given off directly from the aortic trunk : these 

 are the cardiac or coronary arteries. 



Cardiac or Coronary Arteries. (Figs. 258, 259.) 



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

 the tissue of the heart. 



EIGHT CARDIAC ARTERY (Figs. 258, 259, L). This originates from 

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

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

 passing forwards to the right of the pulmonary artery, beneath the anterior 

 auricle ; then to the right and backwards, to reach the auriculo- ventricular 

 groove, which it follows till near the origin of the right ventricular furrow. 

 Here it divides into two branches : one vertical, descending in this furrow 

 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, 

 is smaller than the first, and follows the primitive course of the artery 

 in the auriculo-ventricular groove, also inosculating with the artery of the 

 left side. 



LEFT CARDIAC ARTERY (Fig. 258, 2). This arises opposite the pre- 

 ceding, 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 furrow ; the horizontal is lodged in the coronary 

 groove ; and both anastomose with the analogous branches of the opposite 

 vessel. 



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 which are 

 entirely ventricular ; while from the horizontal circle come the superior or 

 auricular, and inferior or ventricular 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 infundibulum ; its ramifications anastomose with those of a 

 similar branch from the left artery, and in this way establish another com- 

 munication between the two vessels. 



ARTICLE II. POSTERIOR AORTA. 



Course. This artery is a continuation of the aortic trunk, which it 

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

 desciibing a curve whose convexity 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 



