COMPOSITION AND FORMATION OF LYMPH. 475 



and a second smaller right lymphatic duct, which open into the 

 blood-vessels, each on its own side, at the junction of the sub- 

 clavian and internal jugular veins. While the supply of lymph 

 in the lymph-vessels may be considered as being derived ulti- 

 mately entirely from the blood-plasma, it is well to bear in mind 

 that at any given moment this supply may be altered by direct 

 interchange with the plasma on one side and the extravascular 

 lymph permeating the tissue elements on the other. The 

 intravascular lymph may be augmented, for example, by a 

 flow of water from the bloo^-plasma into the lymph-spaces, and 

 thence into the lymph-vessels, or by a flow from the tissue 

 elements into the lymph-spaces that surround them. The lymph 

 movement is from the tissues to the veins, and the flow is main- 

 tained chiefly by the difference in pressure between the lymph 

 at its origin in the tissues and in the large lymphatic vessels. 

 The continual formation of lymph in the tissues leads to the 

 development of a relatively high pressure in the lymph capil- 

 laries, and as a result of this the lymph is forced toward the 

 point of lowest pressure namely, the points of junction of the 

 large lymph-ducts with the venous system. A brief discussion 

 of the factors concerned in the movement of lymph will be found 

 in the section on Circulation. As would be inferred from its 

 origin, the composition of the intravascular 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 propor- 

 tions 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 arti- 

 ficial 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, coagulates when removed 

 from the lymph-vessels. Ordinarily the clotting is slower than 

 with blood, and the clot is not so firm. If, however, tissue extract 

 is added to the lymph, it clots rapidly and firmly. Chemical ex- 

 amination shows that, like the blood, the lymph contains 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- 



