THE THORACIC DUCT. 943 



verteforffi, however, it begins to incline slightly towards the left, and, finally, at about 

 the lower .border of the seventh cervical vertebra it changes its direction somewhat 

 abrupd)', passing upward, forward and to the left, and then downward and forward, 

 thus forming an arch whose convexity is directed upward and whose extremity 

 opens into the subclavian vein. 



The thoracic duct is formed by the union of the right and left lumbar trunks 

 (trunci lumbales) which drain the lumbar nodes. The left trunk, shortly before its 

 union with the right, is usually joined by an unpaired intestinal trunk (truncus intes- 

 tinalis) that drains the coeliac and mesenteric nodes. Just above its commencement 

 the thoracic duct usually, although not always, presents a pyriform enlargement, the 

 receptaculum chyli (cisterna chyli), which extends upward as far as the level of 

 the eleventh thoracic vertebra, and measures from 5-7.5 cm. (2-3 in. ) in length and 

 from 6-8 mm. in diameter. Above the eleventh thoracic vertebra the duct gradually 

 diminishes in calibre until about the middle of its course, where it again enlarges. 

 The thoracic duct possesses few valves in comparison with other lymphatic vessels, 

 those which do occur being frequently insufficient. Its entrance into the subclavian 

 vein, however, is guarded by two well-developed leaflets, which prevent the passage 

 of blood. into the duct. 



Relations. — In its abdominal portion the thoracic duct lies almost in the median 

 line in front of the bodies of the first two lumbar and twelfth thoracic vertebrae, and 

 between the crura of the diaphragm, or under cover of the right crus. Anteriorly, it 

 is in relation with the right side of the abdominal aorta, with the greater azygos vein 

 to the right. 



In its thoracic portion it lies at first within the posterior mediastinum, but 

 above, it enters the superior mediastinum. In the former it lies anterior to the 

 bodies of the eleventh to the fifth thoracic vertebrae, and has in front of it, from 

 below upward, the pericardium, the oesophagus, and the arch of the aorta. The 

 thoracic aorta lies to the left of it, and to the right are the right pleura and the 

 greater azygos vein. The lower right intercostal arteries pass between it and the 

 bodies of the vertebrae, as does also the terminal portion of the hemiazygos vein. 

 In the superior mediastinum it rests upon the lower part of the left longus colli 

 muscle, being separated by it from the bodies of the upper three thoracic vertebrae. 

 Anteriorly, it is in relation with the origin of the left subclavian artery and with the 

 vertebral vein ; to the left is the left pleura and to the right are the oesophagus and 

 the left recurrent laryngeal nerve. 



Its arch is in relation below with the ape.x of the left lung and with the left sub- 

 clavian artery ; to the left and posterior to it is the vertebral vein and to the right 

 and anteriorly are the left common carotid artery, the left internal jugular vein, and 

 the left pneumogastric nerve. 



Tributaries. — In addition to the right and left lumbar and the intestinal trunks 

 by' whose union it is formed, the thoracic duct receives on either side (i) near its 

 origin, a descending trnnk which drains the posterior nodes of the lower six or seven 

 intercostal soaces ; (2) an ascenditig stem from the upper lumbar nodes which trav- 

 erses the crus of the diaphragm and joins the duct at about the level of the ninth or 

 tenth thoracic vertebrae ; (3) the efferent vessels from the upper posterior intercostal 

 nodes, which sometimes unite to form a single ascending stem opening into the upper 

 part of the duct ; (4) the efferent vessels of the posterior mediastinal nodes ; (5) the 

 left jugular trunk ; and, occasionally, (6) the left subclavian and (7) the left broncho- 

 mediastinal trunks, these last three uniting with the duct just before it opens into the 

 subclavian vein. 



Variations. — The thoracic duct is subject to numerous variations, so much so that 

 certain authors have regarded as typical arrangements which others have considered to be 

 abnormal. 



Its origin is frequently opposite the body of the first lumbar vertebra or even 

 opposite the last thoracic ; and rarely it is below the lower border of the second lumbar. 

 Instead of being formed by the union of only two trunks, three are frequently found 

 participating in its origin, the odd one being the intestinal trunk which usually opens into 

 the left lumbar trunk. Occasionally all three trunks are represented by a number of smaller 

 stems which anastomose with one another as well as with the descending stems from the 

 posterior intercostal nodes, the plexus so formed communicating by a number of efferents 



