472 THE ARTERIES 



In addition to the above variations, the arch of the aorta may be higher or lower ia the 

 thorax than normal. In the former ease it may cross the first bone of the sternum, just below, 

 or even a little above, the sternal notch, and reach the side of the second or third thoracic vertebra 

 instead of the fourth. In the latter case, that is, when it is situated lower in the thorax than 

 normal, it may cross the sternum below the level of the manubrium, and come into contact with 

 the spine on a level with the fifth or sixth thoracic vertebra. 



2. The Chief Variations in the Three Primary Branches of the Transverse Portions of 



the Aortic Arch 



(1) All may arise from a common stem : a condition explained on the supposition that the 

 anterior aortic roots have become fused, and have so collected all the branches together. A 

 similar condition is normal in the horse. This variation is rare. 



(2) They may arise as two stems : (a) One stem may be common to the right subclavian, 

 right carotid, and left carotid, the other stem being the left subclavian. This is one of the com- 

 monest variations in the arrangement of the primary branches. It is the normal condition in 

 many mammals, and was formerly described as the normal condition in man. It is of some 

 surgical interest, in that the left carotid when thus given off may cross in front of the trachea. 

 {b)One stem may be common to the left subclavian, left carotid, and right carotid, the other 

 stem being the right subclavian. In this variety the right carotid passes in front of the trachea. 

 (t) There may be two innominate arteries, one stem forming the right, the other a left innominate 

 artery as in birds, (c?) One stem may be common to the right and left carotid arteries, the other 

 stem being the left subclavian— the right subclavian in this case coming from the thoracic aorta. 



(3) They may arise as three stems : but, in place of being given oif in the normal way, (a) 

 the right and left subclavian may come off separately, and the carotids arise by a common stem 

 between the subclavians. This is the normal condition in some cetaceans, (h) The right sub- 

 clavian and right carotid may arise separately, the left carotid and left subclavian forming a left 

 innominate stem, (c) The innominate may give off the left carotid, the left vertebral, and left 

 subclavian, the other two vessels arising from the arch. 



(4) They may arise as four stems, which may be given off in the following order : (a) 

 innominate, left carotid, left vertebral, left subclavian (the commonest condition) ;_ (b) right caro- 

 tid, left carotid, left subclavian, right subclavian ; (c) right subclavian, right carotid, left carotid, 

 left subclavian — the normal condition in the walnis and wombat. 



(5) They may arise as five stems, the five branches being given off in the following order : 

 (a) right subclavian, right carotid, left carotid, left vertebral, left subclavian; (h) right carotid, 

 left carotid, left vertebral, left subclavian, right subclavian; (c) right carotid, left carotid, left 

 subclavian, left vertebral, right subclavian ; {d) right subcla%Han, right external carotid, right 

 internal carotid, left common carotid, left subclavian ; (e) innominate, right vertebral, left caro- 

 tid, left vertebral, left subclavian. 



(6) They may arise as six stems, given off as follows : right subclavian, right vertebral, right 

 carotid, left carotid, left vertebral, left subclavian. 



Branches of the Arch of the Aorta 



1. The ascending portion gives off — fl) Right coronary; (2) left coronary. 



2. The transverse portion gives off — (1) Innominate; (2) left common caro- 

 tid; (3) left sulK'lavian. 



3. The descending portion gives oft' no l)ranch. 



I. Tlie l)ranchi-s of the first, or ascending, ]»ortion of the arch of the aorta are 

 the right and left coronary for the supply of the tissues of the heart. They come 

 off from the aorta, immediately above the aortic valves, from two of the dilatations 

 known as the sinuses of \^alsalva. 



THE RIGHT CORONARY ARTERY 



The right coronary artery (figs. 321 and 326) arises from the right anterior 

 sinus of Valsalva, and, ])assing forwards and t<> the right l)etween the right auricular 

 appendix and the pulmonary artery, turns to the right, and courses in the right 

 auriculo-ventricular groove to the back of the heart, where it follows the posterior 

 interventricular groove to the apex of the ventricles, giving off, however, a small 

 transverse branch, which continues in the groove between the left auricle and ven- 

 tricle to anastomose with the terminal branch of the left cor<^nary artery. In this 

 course it gives off the following branches: (a) A right auricular branch (fig. 

 326), wliich turns backwards and upwards between the right auricle and the aorta, 



