72 AX AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



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 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 endothelial cells composing the capillary walls. A 

 discussion upon these points is in progress 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 investigations. 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 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 

 oeeds of the organs in various organic and inorganic constituents. Thus, 

 to take an illustration that has been much discussed, one kilogram of cows' 

 milk contains 1.7 grams CaO, and the entire milk of twenty-four hours would 

 contain in round numbers 42.5 grams CaO. Since the lymph contains nor- 

 mally about 0.18 parts of CaO per thousand, it would require 236 liters of 

 lymph per day to supply the necessary CaO to the mammary glands. Heiden- 

 hain himself suggests that the difficulty in this case may be met by assuming 

 active diffusion processes in connection with filtration. If, for instance, in the 

 case cited, we suppose that the CaO of the lymph is quickly combined by the 

 tissues of the mammary gland, then the tension of calcium salts in the lymph 

 will be kept at zero, and an active diffusion of calcium into the lymph will occur 

 so long as the gland is secreting. In other words, the gland will receive its 

 calcium by much the same process as it receives its oxygen, and will get its 

 daily supply from a comparatively small bulk of lymph. Strictly speaking, 

 therefore, the difficulty we are dealing with here shows only the insufficiency 

 of a pure filtration theory. It seems possible that filtration and diffusion 

 together would suffice to supply the organs, so far at least as the diffusible 

 substances are concerned. 



2. Heidenhain found that occlusion of the inferior vena cava causes not 

 only an increase in the How of lymph — as might be expected, on the filtration 

 theory, from the consequent rise of pressure in the capillary regions — but also 

 an increased concentration in the percentage of proteid in the lymph. This 

 latter fact has been satisfactorily explained by the experiments of Starling. 2 

 According to this observer, the lymph formed in the liver is normally more 



1 Archie fur die gesammte Physioloffie, 1891, Bd. xlix. S. 209. 

 'Journal of Physiology, 1894, vol. xvi. p. 234. 



