LYMPH. 



363 



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. Lymph 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, espe- 

 cially the fibrinogen. Lymph coagulates, but does so more slowly and less 

 firmly than the blood. Historically, lymph consists of a colorless liquid con- 

 taining a number of leucocytes, and after meals a number of minute fat-drop- 

 lets ; 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 by filtration through the capillary walls. Various condi- 

 tions which 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 support such a 

 theory, inasmuch as the inorganic salts contained in it are in the same concen- 

 tration, approximately, as in blood-plasma, while the proteids are in less con- 

 centration, following the well-known law that in the filtration of colloids 

 through animal membranes 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 filtration alone 

 does not suffice to explain the composition of the lymph under all circum- 

 stances. At present two divergent views are held upon the subject. Accord- 

 ing to some physiologists, all the facts known with regard to the composition 

 of lymph may be satisfactorily explained if we suppose that this liquid is 

 formed from blood-plasma by the combined action of the two physical pro- 

 cesses of filtration and diffusion. According to others, it is believed that, in 

 addition to filtration and diffusion, it is necessary to assume an active secretory 

 process on the part of the endothelial cells composing the capillary walls. A 

 discussion upon these points is in progress at present in current physiological 

 literature, and it is impossible to foresee definitely what the outcome will be, 

 since a final conclusion can be reached only by repeated experimental investi- 

 gations. The actual condition of our knowledge of the subject can be presented 

 most easily by briefly stating the objections which have been raised by Heiden- 

 hain 1 to a pure filtration-and-diffusion theory, and indicating how these objec- 

 tions have been met. 



1. Heidenhain shows by simple calculations that an impossible formation 

 of lymph would be required, upon the filtration theory, to supply the chemical 

 1 Archiv fur die gesammte Physiologic, 1891, Bd. xlix. S. 209. 



