336 THE AORTIC ARCH. 



to the left, and thus the aorta comes into view. It approaches very near to 

 the sternum, from which it is separated only by the pericardium, by some 

 connective tissue, and by the remains of the thymus gland lodged in the 

 mediastinal space : higher up, the descending vena cava lies on the right 

 side, and the pulmonary artery passes backwards on the left ; while behind 

 are placed the right branches of the pulmonary vessels. 



The second or transverse part of the arch is covered on the left side by 

 the left pleura and lung, and is placed immediately in front and to the 

 left of the trachea, before its bifurcation into the bronchi : it touches like- 

 wise the oesophagus posteriorly. The upper border of the transverse part 

 of the arch has in contact with it the left innominate vein ; and from 

 it are given off the large arteries (innominate, left carotid, and left sub- 

 clavian), which are furnished to the head and the upper limbs. The lower 

 or concave border overhangs the bifurcation of the pulmonary artery, and 

 is connected with the left branch of that artery by the remains of the 

 ductus arteriosus. At or near its end this part of the arch is crossed in 

 front by the left vagus and phrenic nerves, with some offsets of the sympa- 

 thetic ; and the recurrent laryngeal branch of the vagus turns upwards 

 beneath and behind it. 



The descending portion of the arch rests against the left side of the body 

 of the third dorsal vertebra, and is covered by the left pleura. To the 

 right side of this part of the arch is the oesophagus with the thoracic duct. 



BRANCHES. The branches given off from the arch of the aorta are five in 

 number. Two of these, named the coronary or cardiac arteries, com- 

 paratively small, arise from two of the sinuses of Valsalva, and are distri- 

 buted to the walls of the heart. The other three are large primitive trunks, 

 which supply the head and neck, the upper limbs, and, in part, the thorax, 

 and usually arise from the middle or highest part of the arch, in the follow- 

 ing order : first, the innominate or brachio-cephalic artery, which soon sub- 

 divides into the right subclavian and the right carotid arteries ; second, the 

 left carotid ; and, third, the left subclavian artery. The origin of the left 

 carotid artery is usually somewhat nearer to the innominate artery than it is 

 to the subclavian artery of its own side. 



PECULIAKITIES. (For more extended information on the peculiarities of the aorta 

 and of the blood-vessels in general, the student is referred to " The Anatomy of the 

 Arteries/' by Richard Quain, London, 1844.) 



I. Peculiarities of the Arch itself. 



Variations in height The arch sometimes reaches very nearly as high as the top 

 of the sternum. Occasionally it has been found an inch and a half below that level, 

 and in rare instances as much as three inches below it. 



Double arch. This very rare variety has been known to occur in two forms. In 

 both of these the trachea and oesophagus passed through the circle formed by the two 

 divisions of the arch, which united behind them. In one form the arch retained its 

 inclination to the left side ; the pulmonary artery, placed in its proper position, was 

 united to the left division of the aorta by the obliterated ductus arteriosus ; and 

 from each division of the arch two branches arose, the carotid and subclavian. In 

 the other form a symmetrical ring was completed by the two divisions, each giving 

 rise to three vessels subclavian, and external and internal carotid ; while the pul- 

 monary artery dipped into the circle from above, and sent out its branches beneath it. 



Right arch. Arching of the aorta to the right side has been observed to occur in 

 three different forms. 1st. Accompanying total transposition of the heart and 

 viscera. 2nd. Occurring without transposition of other parts, and with the left in- 

 nominate artery, right carotid, and right subclavian given off in succession. 3rd. 

 Occurring, in like manner, without transposition of other parts, its first branch being 



