472 BLOOD AND LYMPH. 



tive liquids of the body, such as the pericardial liquid, the syno- 

 vial liquid, the aqueous humor, the cerebrospinal liquid, etc., 

 which are sometimes classed under the general term "lymph," may 

 vary greatly; thus, the cerebrospinal liquid possesses no morpho- 

 logical elements, contains no fibrinogen, and, therefore, does not 

 clot, and, indeed, has only minute traces of protein of any kind. 



Formation of Lymph. — The careful researches of Ludwig and 

 his pupils were formerly beUeved 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 proteins 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 

 filtration alone does not suffice to explain the composition of the 

 lymph under all circumstances. At present two divergent views 

 are held upon the subject. According 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 physical processes 

 of filtration, diffusion, and osmosis. 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 endothe- 

 lial cells composing the capillary walls. The actual condition of our 

 knowledge of the subject can be presented most easily by briefly 

 stating some of the objections that have been raised by Heidenhain* 

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

 objections have been met. 



1. Heidenhain showed by simple calculations that an impossible 

 formation of lymph would be required, upon the filtration theory, 

 to supply the chemical needs of the organs in various organic and in- 

 organic constituents. Thus, to take an illustration that has been 

 much discussed, one kilogram of cows' milk contains 1.7 gms. CaO 

 and the entire milk of twenty-four hours would contain, in round 

 * "Archiv f. die gesammte Physiologie," 49, 209, 1891. 



