364 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

 take an illustration which has been much discussed, one kilogram of cow's 

 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. Cohnstein 1 has 

 answered the problem in another way. He calls attention to the fact that 

 in the body the capillaries contain blood under a comparatively high pres- 

 sure, while on their exterior they are bathed with lymph, also under pres- 

 sure, although less than that of the blood. The pressure causing filtration 

 in this case is the difference in pressure between the inside and the outside 

 liquid. Moreover these liquids differ in composition, so that diffusion must 

 also take place in such a manner that crystalloids will diffuse out into the 

 lymph, and an amount of water corresponding to the osmotic equivalent will 

 pass into the blood. The lymph that is actually formed will therefore be the 

 balance between these two processes, and a liquid produced in this way he 

 designates specifically as a transudation. From laboratory experiments made 

 with ureters and veins he shows that the percentage composition of the transu- 

 dation in crystalloid substances will increase with the pressure of the outside 

 liquid. As this pressure is raised the filtration-stream is diminished, but the 

 diffusion is unaffected, hence the transudation will be more concentrated. It 

 is possible in this way, as he shows by experiment, to get a transudation much 

 more concentrated than the original liquid, and he assumes that in the body 

 the lymph formed in the tissues may be more concentrated than the blood, and 

 thus a small quantity of lymph may transport a large amount of crystalloid 

 substance. What seems to be a fatal objection to this reasoning, so far as it 

 applies to the difficulty first suggested with regard to the chemical needs of the 

 organs, is the time element. As Heideuhain points out, the more concentrated 

 the transudation the less its bulk, so that to get the required amount of CaO, 

 for example, would upon this hypothesis require much more than twenty- 

 four hours. Strictly speaking, however, 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. Heideuhain found that occlusion of the inferior vena cava causes not 

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

 1 Archiv fur die gesammte Physiologic, 1894, Bd. lix. S. 350. 



