Mediastinum. 147 



border of the second rib, and, therefore, lies behind the 

 second right intercostal space. This vessel is almost en- 

 tirely within the pericardium, and, when distended, is dis- 

 tant from the sternum about one-quarter of an inch. The 

 arch or transverse aorta passes, with a gentle curve, back- 

 wards from the level of the upper border of the second 

 right cartilage, at its junction with the sternum, to the left 

 feide of the fourth dorsal vertebra, behind. This portion 

 of the aorta is about two inches in length, and the convex- 

 ity spoken of is directed upwards and to the right. In its 

 course backwards, it lies behind the first piece of the ster- 

 num, and the highest point of the vessel is about one inch 

 below the upper border of that bone. The thoracic aorta 

 begins where the arch ends, viz., at the lower border of 

 the fourth dorsal vertebra, and passes downwards in the 

 posterior mediastinum to become the abdominal aorta 

 after piercing the diaphragm in front of the twelfth dor- 

 sal vertebra. 



From the transverse part of the arch are given off the 

 large vessels that supply the head, neck and upper ex- 

 tremities, viz., the innominate, the left common carotid 

 and the left subclavian arteries. These vessels arisq 

 from before, backwards, in the order named, and, so close 

 are their origins to one another, that, if we removed the 

 inferior portion of the arch and viewed the orifices of the 

 branches, from below, it would be seen that they were sepa- 

 rated, from each other, by a thin strip only, of the aortic 

 wall. The innominate is about one and a half inches long 

 and passes upwards, a little forwards and to the right, to 

 terminate at the upper limit of the right sterno-clavicular 

 articulation, where it divides into the right common carotid 

 and the right subclavian. The left common carotid, about 

 one and three-quarter inches in length, arises close to the 

 left of the innominate and ascends, obliquely, to the upper 



