508 THE LYMPHATIC VESSELS. [BOOK n. 



body join either the great thoracic duct which opens by a valvular 

 orifice into the venous system at the junction of the left jugular 

 and subclavian veins, or the small right lymphatic trunk which 

 similarly opens into the junction of the right jugular and sub- 

 clavian veins. The latter course is taken by the lymphatics of 

 the right side of the head and neck, the right arm, the right side 

 of the chest, the right lung and the right side of the heart, as well 

 as by some vessels coming from part of the upper surface of the 

 liver ; all the rest of the lymphatics including the lacteals fall into 

 the thoracic duct. 



The lymphatic vessels, while like the veins they join in their 

 course into larger and larger trunks, do not increase in calibre so 

 rapidly or so regularly as do the veins ; they may run for some 

 distance without greatly increasing in size ; and further they, 

 unlike the veins, freely anastomose, forming plexuses. Moreover 

 during their course they enter into peculiar relations with struc- 

 tures known as lymphatic glands. 



It will be advantageous to consider separately the lymphatic 

 vessels other than the lymph-capillaries, the lymph-capillaries 

 themselves, and the lymphatic glands. 



The Lymphatic Vessels. 



286. On these we need not dwell at length since their 

 structure, in its main features, resembles that of the veins. The 

 thoracic duct, which in man has at its lower end where it is 

 widened into what is sometimes called the receptaculum chyli a 

 diameter of six or seven millimetres, but is narrower higher up, 

 may be said to possess three coats. The inner coat consists of a 

 layer of fusiform epithelioid cells, not unlike those in a vein but 

 more elongated and with a tendency to be sinuous in outline, and 

 of a slender elastic lamina on which these rest. The middle coat 

 consists of fine bundles of plain muscular fibres, which are for the 

 most part disposed circularly but also to a certain extent obliquely 

 and even longitudinally. The spaces between the bundles of muscu- 

 lar fibres are occupied by connective tissue and networks of elastic 

 fibres. The outer coat, which is not well defined either from the 

 middle coat on the one side or the connective tissue surrounding 

 the duct on the other side, consists chiefly of connective tissue 

 with elastic elements, a few muscular fibres being sometimes 

 present. The wall of the thoracic duct is essentially muscular, 

 and from the scantiness of connective tissue and of elastic elements 

 is more tender, more apt to be torn than the wall of a vein of 

 corresponding size. Numerous valves are present, these like the 

 valves of the veins being foldings of the inner coat. 



The smaller vessels resemble in structure the thoracic duct, 

 the coats being of course more slender. In the majority of even 

 smaller lymphatic vessels the muscular fibres are abundant. 



