350 THE HUMAN BODY. 



and chemical (pp. 49, 62), which justify us in speaking of it 

 in general as the lymph. The lymphatic vessels collect this 

 lymph or at least such part of it as does not pass back locally 

 by diffusion into the blood, and pour it into the veins. 



The Structure of Lymph-vessels. The smallest lymph- 

 vessels proper are the lymph -capillaries; tubes rather wider 

 than the blood-capillaries, but like them having a wall con- 

 sisting of a single layer of flattened epithelium cells. The 

 cells have, however, a wavy margin and are not as a rule much 

 longer in one diameter than another, in both of which respects 

 they differ from the cells of the corresponding blood-vessels. 

 In some regions, as in many glands, the lymph-capillaries are 

 much dilated and form irregular lymph lacunce, everywhere 

 bounded by their peculiar wavy cells, lying in the interstices 

 of organs; and sometimes they form tubes around small blood- 

 vessels, as in the brain (perivascular lympJt-channel). In 

 some places they commence by blind ends as in the lacteal 

 vessels of the villi of the small intestine (Fig. 115) which are 

 lymph-capillaries; but usually they branch and join to form 

 networks. Lymph from the canaliculi enters them (whether 

 by passing through their boundary cells or through clefts left 

 between these is not certain) and is passed on to larger vessels 

 which much resemble veins of corresponding size, having the 

 same three coats, and being abundantly provided with valves. 



The Thoracic Duct. The lymph-vessels proceeding from 

 the capillaries in various organs become larger and fewer by 

 joining together, and all end finally in two main trunks which 

 open into the venous system on the sides of the neck, at the 

 point of junction of the jugular and subclavian veins. The 

 trunk on the right side is much smaller than the other and 

 is known as the " right lymphatic duel." It collects lymph 

 from the right side of the thorax, from the right side of the head 

 and neck, and the right arm. The lymph from all the rest 

 of the Body is collected into the thoracic duct. It com- 

 mences at the upper part of the abdominal cavity in a dilated 

 reservoir (the receptaculum cht/li), into which the lacteals 

 from the intestines, and the lymphatics of the rest of the 

 lower part of the Body, open. From thence the thoracic 

 duct, receiving tributaries on its course, runs up the thorax 

 alongside of the aorta and, passing on into the neck, ends on the 

 left side at the point already indicated; receiving on its way 

 the main stems from the left arm and the left side of the 



