382 THE HUMAN BODY 



lation can be drained away from the tissues and carried back to 

 the blood. This drainage is afforded by the lymphatic system. At 

 its beginning this system is without definite structure, consisting 

 simply of intercellular spaces. These communicate with one an- 

 other, and at intervals with minute vessels having definite walls. 

 These latter are the beginnings of definite lymph-channels. 



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 consisting 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 lacunw, 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 lymph- 

 channel). In some places they commence by blind ends as in the 

 lacteal vessels of the villi of the small intestine (Fig. 131) which 

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

 networks. Lymph from the intercellular spaces enters them 

 (probably by passing through their boundary cells) 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 

 duct. 11 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 

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

 reservoir (the receptaculum chyli), 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, pass- 



