COMPOSITION AND FORMATION OF LYMPH. 471 



There is a similar but much smaller lymphatic trunk, the right 

 lymphatic duct, on the right side which gathers the lymph from 

 the right side of the head, the right arm, and the right thorax. 

 The flow of lymph is a slow stream beginning back in the tissues 

 where the tissue fluid is under some pressure, and ending where the 

 thoracic duct and the right lymphatic duct open into the veins. 

 The system of lymph vessels acts as a drainage system which aids 

 in maintaining the water content of the body at a normal level. 

 While the supply of lymph is derived ultimately from the blood- 

 plasma, it is to be borne in mind that the supply round the tissue- 

 cells, the tissue fluid, can interchange its water and dissolved sub- 

 stances in three directions. By osmosis and diffusion there may 

 be an interchange between the tissue fluid and the blood-plasma 

 through the walls of the blood capillaries, or between the tissue 

 fluid and the lymph in the lymph capillaries through the walls of 

 the latter, or finally, between the tissue fluid and the large volume 

 of liquid in the tissue cells. Through these various interchanges 

 a certain balance or equilibrium is established by means of which 

 the water contents of the tissues ip maintained. A disturbance in 

 this equilibrium may le; d to an a 'cumulation of tissue fluid, giv- 

 ing rise to the condition known as edema or dropsy. 



As would be inferred from its origin, the composition of the in- 

 travascular lymph is essentially the same as that of blood-plasma. 

 It contains the three blood proteins, the extractives (urea, fat, 

 lecithin, cholesterin, sugar), and inorganic salts. The salts are 

 found in the same proportions as in the plasma; the proteins are 

 usually stated to be less in amount. Histological examination 

 shows that the lymph contains many leucocytes almost all of 

 which belong to the group of small lymphocytes. Red corpuscles 

 are frequently found, especially when the flow of lymph has been 

 stimulated by artificial means. It is believed usually that these 

 red corpuscles are not a normal or constant element of the lymph, 

 but represent, so to speak, an accidental constituent. When red 

 corpuscles for any reason get through the capillary walls into the 

 tissue spaces they are gathered up in the lymph-stream. Blood- 

 platelets seem to be entirely lacking. Lymph, like blood, coagu- 

 lates when removed from the lymph-vessels. Ordinarily the clot- 

 ting is slower than with blood, and the clot is not so firm. If, how- 

 ever, tissue extract is added to the lymph, it clots rapidly and firmly. 

 Chemical examination shows that, like the blood, the lymph con- 

 tains fibrinogen, prothrombin, antithrombin, and calcium salts. 

 When collected without contamination with tissue juice, its slower 

 and feebler clotting as compared with the blood may be explained by 

 the absence of the blood-platelets. The composition of the exuda- 



