362 THORACIC DUCT. 



These vessels generally unite on the second or third lumbar 

 vertebrae ; and, in some instances, the trunk which they form 

 dilates considerably, soon after its commencement ; in conse- 

 quence of which it was formerly called the receptacle of the 

 chyle. At first it lies behind the aorta, but it soon inclines to 

 the right of it, so as to be behind the right crus of the diaphragm. 

 In the thorax, it appears on the front of the spine, between the 

 aorta and the vena azygos, and continues between these ves- 

 sels until it has arrived at the fourtli or third dorsal vertebra. 

 It then inclines to the left, and proceeds in that direction until 

 it emerges from the thorax, and has arisen above the left 

 pleura, when it continues to ascend behind the internal jugular, 

 nearly as high as the sixth cervical vertebra : it then turns 

 downward and forward, and after descending from six to ten 

 lines, terminates in the back part of the angle formed by the 

 union of the left internal jugular with the left subclavian vein. 

 Sometimes, after rising out of the thorax, it divides into two 

 branches, which unite before they terminate. Sometimes 

 it divides, and one of the branches terminates at the above 

 mentioned angle, and the other in the subclavian vein, to the 

 left of it. 



The orifice of the thoracic duct has two valves, which 

 effectually prevent the passage of blood into it from the system 

 of the vena cava. 



There are sometimes slight flexures in the course of the duct ; 

 but it generally inclines to the left, in the upper part of the 

 thorax, as above mentioned ; and is then so near the left lamen 

 of the mediastinum that, if it be filled with colored injection, 

 it can be seen through that membrane, when the left lung is 

 raised up and pressed to the right. 



The duct sometimes varies considerably in its diameter in 

 different parts of its course. About the middle of the thorax 

 it has often been found very small. In these cases it generally 

 enlarges in its progress upwards, and is often three lines in 

 diameter, in its upper part. Many anatomists have observed 

 it to divide and to unite again, about the middle of the thorax. 



