382 THE CORONARY ARTERIES. 



backwards on the left, it comes very near the sternum, from which it is separated 

 by the pericardium, by the right pleura with the narrow portion of the anterior 

 mediastinum and a little fat, and to a variable extent by the thin anterior margin 

 of the right lung. On its right side are the superior vena cava and the right 

 auricle ; on the left side is the pulmonary trunk ; and behind are placed the left 

 auricle of the heart and the right pulmonary artery. 



BRANCHES. The only branches of the ascending aorta are the coronary arteries, 

 two comparatively small vessels for the supply of the heart. They arise from the 

 root of the aorta, in the upper parts of two of the sinuses of Valsalva, and about on a 

 level with the free margins of the corresponding semilunar flaps of the aortic valve. 



The right coronary artery (iv) arises from the anterior sinus of Valsalva. 

 It comes forwards between the pulmonary artery and right auricular appendix, then 

 runs obliquely in the auriculo^ventricular groove on the right side and the posterior 

 aspect of the heart, until it reaches the line of separation between the two ventricles, 

 where it divides into two branches. The smaller of these (transverse) continues 

 transversely in the groove between the left auricle and ventricle, approaching the 

 termination of the transverse branch of the left coronary artery ; while the other 

 branch (descending) runs longitudinally downwards along the posterior interventri- 

 cular groove, giving branches to each ventricle and to the septum between them. 



In its course the right coronary artery gives, besides the offsets already noticed, 

 small branches to the right auricle and ventricle, and also to the first part of the 

 aorta and the pulmonary trunk. An infundibular branch ramifies over the front of 

 the conus arteriosus of the right ventricle, and a larger marginal branch descends 

 along the right border towards the apex of the heart, giving offsets to the anterior 

 and posterior surfaces of the ventricle. 



The left coronary artery (iv) is generally rather larger than the preceding, and 

 arises from the left sinus of Valsalva. It passes behind and then to the left side of 

 the pulmonary trunk, appearing between that vessel and the left auricular appendage, 

 where it divides into two branches. Of these, the one (posterior or transverse) runs 

 in the groove between the left ventricle and auricle, and approaches at the posterior 

 aspect of the heart the transverse branch of the right coronary artery ; the other 

 branch (anterior or descending), much the larger, descends along the anterior inter- 

 ventricular groove, to the right of the apex of the heart. 



The left coronary artery supplies at its beginning small branches to the pulmon- 

 ary artery, ascending aorta, and the left auricle. Its anterior division furnishes 

 offsets to both ventricles and the interventricular septum ; while from its posterior 

 division small branches pass to the left auricle and ventricle, and a considerable 

 marginal branch usually runs along the left border of the heart posteriorly. 



The two coronary arteries have fine anastomoses with one another on the surface 

 of the heart, and they communicate also, by means of the small branches given to 

 the coats of the great vessels, with the pericardial and bronchial arteries. 



Varieties. The level of the orifices of the coronary arteries in the sinuses of Valsalva 

 varies somewhat, being 1 often above, and sometimes below the free margin of the valve- 

 segment. The two arteries have been seen commencing by a common trunk, or both arising 

 from one sinus of Valsalva. The existence of three arteries is not unfrequent, and in a few 

 instances four have been observed, the supplementary vessels being generaMy small, springing 

 from the aorta near the main coronary trunk, and representing normal branches of the 

 latter which have acquired an independent origin : the two divisions of the left artery 

 occasionally come off separately from the left sinus of Valsalva. In some very rare cases an 

 additional coronary artery has been found arising from the pulmonary trunk (W. Krause ; 

 H. St. John Brooks, two cases, Journ. Anat., xx). 



A diminution in the size of one coronary artery is compensated by an increase in the 

 other, which is then distributed over a larger area, especially on the back of the heart : the 

 left artery may thus furnish the branch descending in the posterior interventricular furrow. 



