116 THE ANATOMY OF THE HORSE. 



large arteries in the anterior mediastinum. It is formed at the entrance 

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

 sides, and it terminates in the right auricle. It is better seen on the 

 right side of the thorax, and will be more fully described in that con- 

 nection. It receives the following branches on this side : — 



1. The Internal Thoracic, which accompanies, and exactly corresponds 

 to, the homonymous artery. 



2. A large venous trunk formed by the union of the vertebral, superior 

 cervical, and dorsal veins. It crosses to the left of the axillary artery to 

 reach the vena cava. Exceptionally, as in Plate 23, the vertebral vein 

 may join the vena cava independently. The vertebral and superior 

 cervical veins exactly correspond to the arteries of the same names, but 

 the subcostal root of the dorsal vein is of greater extent than the corre- 

 sponding artery, for it drains the intercostal spaces from the 3rd to the 

 11th or 12th. The left dorsal vein is also called the small vena azygos. 

 These veins are superficially placed to the corresponding arteries as they 

 lie on the oesophagus and longus colli. < 



Intercostal Veins. — The last five or six of the left side join the great 

 vena azygos, a vessel of the right side of the chest. 



The Thoracic Duct. This is the largest lymphatic vessel in the 

 body, and has a calibre about twice that of a goose quill. It will be 

 most readily found in the angle of separation of the anterior and 

 posterior aorta?, resting on the trachea, at the lower edge of the 

 oesophagus. It will be recognised as a very thin-walled vessel, empty 

 or with a small amount of coloured contents, so that it might be mis- 

 taken for a vein. There is not, how r ever, any vein of so large a size in 

 this situation. Open it, and pass a blunt probe along it towards the 

 entrance of the chest. Most commonly the duct has the following 

 course : Entering the chest by the hiatus aorticus, to the right of the 

 posterior aorta, it extends forwards along the spine, having the aorta on 

 its left and the great vena azygos on its right. It descends from the spine 

 on the right side of the aortic arch, and crosses the before-mentioned 

 angle, where it rests on the trachea. It then passes to the right side of 

 the left axillary artery, and dips down between that vessel and the 

 arteria innominata. It terminates at the anterior edge of the 1st rib, 

 in the angle of junction of the left jugular with its fellow or with the 

 left axillary vein, that is, at the beginning of the anterior vena cava. 

 At its termination it is slightly dilated, and furnished with a valve. 

 The duct may be found to the left of the posterior aorta, or it may be 

 double as far as the heart, there being a branch on each side of the 

 aorta. The thoracic duct discharges into the venous system the 

 lymph collected throughout the whole animal except the right fore 

 limb, and the right side of the head, neck, chest-wall, and diaphragm. 

 It is by this channel, also, that the chyle absorbed from the intestine 



