120 THE ANATOMY OF THE HORSE. 



convexity of the aortic arch, a little anterior to the bifurcation of the 

 trachea. Generally, as in Plate 24, it arises not independently but as 

 a division of a short vessel which is at the same time the common 

 trunk for the 1st and 2nd pairs of aortic intercostals. It is a small 

 vessel, not larger than an intercostal. It is reflected downwards and 

 backwards on the right side of the aorta, and divides into the hronchial 

 trunk and the (esophageal artery. The oesophageal artery, which is the 

 smaller of the two, is continued backwards above the gullet, through 

 the posterior mediastinum, extending sometimes to near the foramen 

 sinistrum, and anastomosing with the pleuro-oesophageal branch of the 

 gastric artev}-. Sometimes there is an analogous vessel in the mediasti- 

 num below the oesophagus (inferior oesophageal), but when present this 

 is a very slender artery. The brotickial trunk dips down between the 

 aorta and the gullet, and bifurcates to form the right and left bronchial 

 arteries, each of which enters the root of the lung on the bronchus. It 

 is the nutrient vessel to the lung. The above-mentioned inferior 

 ossophageal may be, a branch of one of the bronchial arteries. 



The Arteeia Innominata is the right division of the anterior aorta, 

 the left axillary artery Ijeing the other division. In calibre it is greater 

 than the, left axillary, and it is placed on a lower level. It is related to 

 the trachea above, and to the anterior vena cava below and to the left. 

 After a course of about two inches it divides to form the cephahc trunk 

 and the right axillary artery, and immediately in front of its point of 

 division it detaches the dorse- cervical artery. 



The DoRSO-CERViCAL Artery. This is a short trunk which passes 

 upwards on the trachea, and divides to form the dorsal and superior 

 cervical arteries, which have precisely the same course and distribution 

 as those of the left side. They have also the same connections save that 

 they do not touch the oesophagus. 



The Cephalic Artery. This vessel, which has a length of about two 

 or three inches, passes directly forwards at the lower face of the trachea, 

 and bifurcates at the entrance to the chest, forming the common carotid 

 arteries (right and left). 



The Kight Axillary Artery. This vessel in its intrathoracic 

 course forms a continuous curve, or arch, with the arteria innominata ; 

 this arch being, however, on a lower level, and less abrupt, than that 

 of the left axillary. It gives off here the vertebral and internal thoracic 

 arteries. 



The Vertebral Artery and the Internal Thoracic Artery do not 

 differ from the homonymous vessels of the left side. 



The Anterior Vena Cava. This large vessel, already referred to, is 

 best seen from the right side of the chest. It is formed at the entrance 

 to the chest by the union of the jugular and axillary veins of both sides, 

 and its initial portion is fixed by fibrous processes to the inner surfaces 



