THORAX AND ABDOMEN OF THE HORSE 55 



The vena azygos receives the last twelve or tliirteen intercostal 

 veins from the right side of the thorax, and the last five or six (possibly 

 more) from the left side. Sometimes the left intercostal veins join a 

 small vessel (v. hemiazygos) that follows the left dorsal border of the 

 aorta for a distance, then crosses the dorsal face of this artery, and 

 finally unites with the vena azygos. 



Small oesophageal and bronchial veins empty their blood into the 

 vena azygos as this vessel is crossing the oesophagus and trachea. 



Ductus thoracicus. — By means of the thoracic duct the lymph 

 from the whole body, with the exception of that from the right side of 

 the thorax, the rigfht thoracic limb, and the right side of the head and 

 neck, gains the venous system. Consequently, the duct is the largest 

 lymphatic vessel in the body. 



The thoracic duct begins in the abdomen, on a level with the first 

 and second lumbar vertebrae and between the aorta and the right crus 

 of the diaphragm, as an irregular, elongated dilatation known as the 

 cisterna chyli} It enters the thorax to the right of the aorta, between 

 this vessel and the vena azygos. 



The thoracic part of the duct is subject to a considerable amount of 

 variation. For a longer or shorter distance there may be two ducts, 

 right and left. The right duct — often the only one — runs between the 

 aorta and the vena azygos to about the sixth thoracic vertebra. Here 

 it inclines to the left, and, sloping in a ventral direction, crosses the left 

 face of the oesophagus and trachea. Then, running to the right of the 

 left subclavian artery, it terminates, somewhat dilated, on a level with 

 the cranial border of the first rib byjoining either the commencement 

 of the cranial vena cava or the short common vein produced by the 

 union of the two jugulars. The opening of the duct into the vein is 

 provided with a valvular arrangement for the purpose of preventing 

 entrance of blood into the duct. It commonly happens, however, that a 

 small amount of blood eludes the valve and stains the contents of the 

 duct for some little distance. 



The left duct is very frequently absent, either wholly or in part. 

 At its greatest development it runs along the vertebral bodies to the 

 left of the aorta as far as the sixth thoracic vertebra, where it joins the 

 right duct. If the two ducts are both present, frequent communications 

 are established between them. 



Ductus lymphaticus dexter. — The lymph from those parts of the 



body not drained by the thoracic duct finds its way to the venous 



system by way of the right lymphatic duct. This is a short vessel (4 or 



5 cm. in length), not wider than a goose-quill, which terminates in a 



1 XvKos (chylos) [Gr.], juice. 



