428 BLOOD AND LYMPH. 



be considered as being derived ultimately 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 extra vascular lymph permeating the tissue elements on 

 the other. The in tra vascular lymph may be augmented, for ex- 

 ample, by a flow of water from the plasma into the lymph spaces, 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 maintained chiefly by the difference in pres- 

 sure 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 

 capillaries, 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 fuller 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 lymph is essentially the same as that of blood- 

 plasma. It contains the three blood proteids, the extractives (urea, 

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

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

 less in amount, especially the fibrinogen. Lymph coagulates, but 

 does so more slowly and less firmly than the blood. Histologically , 

 lymph consists of a colorless liquid containing a number of leuco- 

 cytes, and after meals a number of minute fat droplets; red blood 

 corpuscles occur only accidentally, and blood plates, according to 

 most accounts, are likewise normally absent. 



Formation of Lymph. The careful researches of Ludwig and 

 his pupils were formerly believed to prove that the lymph is derived 

 directly from the plasma of the blood mainly by filtration through 

 the capillary walls. Emphasis was laid on the undoubted fact that 

 the blood within the capillaries is under a pressure higher than that 

 prevailing in the tissues outside, and it was supposed that this excess 

 of pressure is sufficient to squeeze the plasma of the blood through 

 the very thin capillary walls. Various conditions that alter the 

 pressure of the blood were shown to influence the amount of lymph 

 formed in accordance with the demands of a theory of filtration. 

 Moreover, the composition of lymph as usually given seems to sup- 

 port such a theory, inasmuch as the inorganic salts contained in it 

 are in the same concentration, approximately, as in blood-plasma, 

 while the proteids are in less concentration, following the well- 

 known law that in the filtration of colloids through animal mem- 

 branes the filtrate is more dilute than the original solution. This 

 simple and apparently satisfactory theory has been subjected to 

 critical examination within recent years, and it has been shown that 



