ENDING OF THORACIC DUCT. 119 



the scalenus, are nearly the same as on the right side (p. 76); but the 

 student must note for himself the variations that may exist in the connec- 

 tions. 



The branches of this artery resemble so closely those of the right trunk, 

 that one description will serve for both (p. 76 to 78). It may be re- 

 marked, that the superior intercostal of the left side is usually internal to, 

 instead of beneath the scalenus as on the right side; in other words, this 

 branch arises sooner. 



The thoracic duct (fig. 27, 8 ) conveys the chyle and lymph of the 

 greater part of the body into the venous circulation. Escaping from the 

 thorax on the oesophagus, the duct ascends in the neck as high as the 

 seventh or sixth cervical vertebra. At the spot mentioned it issues from 

 beneath the carotid trunk, and arches outwards above or over the subcla- 

 vian artery, and in front of the scalenus muscle and the phrenic nerve, to 

 open into the subclavian close to the union with the internal jugular vein. 

 Double valves, like those of the veins, are present in the interior of the 

 tube ; and a pair guards the opening into the posterior part of the vein, 

 to prevent the passage of the blood into it. Frequently the upper part of 

 the duct is divided; and there maybe separate openings into the large 

 vein, corresponding with those divisions. 



Fig. 27. 



DIAGRAM OF THE ENDING OF THE LYMPH DUCT AND THE THORACIC DUCT IN THE VEINS. 



1. Upper veua cava. 8. Thoracic duct. 



2. Right, and 3, left innominate vein. 9. A lymphatic vessel joining the right lym- 

 4. Left, and 5, right internal jugular. phatic duct, as thi's is about to end in 

 6. Left, and 7, right subclavian vein. the subclavian vein. 



Large lymphatic vessels from the left side of the head and neck, and 

 from the left upper limb, open into the upper part of the duct, and some- 

 times separately into the vein ( 10 ). 



Structure. This tube is formed of three coats like the bloodvessels, 

 viz., inner, middle, and outer. The inner is an elastic layer of longitudi- 

 nal fibres covered by flattened epithelium; the middle is muscular and 

 elastic with transverse fibres ; and the outer is constructed chiefly of fibrous 

 tissues arranged longitudinally and obliquely. 



Examine next the brachial and cervical plexuses, using the description 

 of the right side (p. 79 to 81). 



