THE VASOMOTOR MECHANISM 345 



doubtless have much to do with the phenomenon. These, how- 

 ever, are factors which physiology at present is unable to ana- 

 lyze completely, so that the vasomotor theory affords our most 

 satisfactory explanation of sleep from the physiological standpoint. 



The Lymphatics. The living cells of the Body, as previously 

 pointed out (Chap. XVII), are bathed in lymph, a liquid derived 

 from the blood and serving as the intermediary by which inter- 

 changes of food materials, gases and waste substances between 

 it and the cells are carried on. At the same place it was shown 

 that there is a continuous movement of liquid from the blood into 

 the lymph spaces, necessitating a system whereby the accumu- 

 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 an- 

 other, 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, eveiywhere 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. 132) 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 



