272 THE BLOOD AND THE LYMPH 



so extremely permeable, the two-thirds-liquid tissues are made practically 

 one, save as the selective powers of the endothelium and other proto- 

 plasm concerned alter the intake and the output to suit their own local 

 demands. The blood rushing everywhere through the body, the ever- 

 intervening lymph, and the living tissues form together a unity of 

 semi-liquid protoplasm too perfect and too complicated in its interactions 

 for us at present to understand it completely. In this unity and in these 

 interactions of internal nutrition the lymph plays the very important part 

 now sufficiently indicated. (See also below, p. 385.) 



The forces which cause the passage back and forth out of and into the 

 blood-capillaries, the lymph-spaces, and lymph-capillaries need not be 

 gone over, for we have already seen practically the same process occurring 

 in the lungs, in the gut-wall, in the kidneys, etc. It is apparently a 

 complex event, partly filtration, diffusion, and osmosis, and partly a se- 

 lective, physical sort of secretion. As we have said before, it is a sort of 

 osmosis doubtless, but it is osmosis through an animal membrane which 

 is alive and which selects what shall pass through it. This selection is 

 probably determined more by the chemical composition of the liquids 

 which pass back and forth than merely by hydraulic principles. Perhaps 

 the ionic dissociation of the salines and of the organic crystalloids is 

 especially important. We do not know to how great an extent by means 

 of enzymes or ferments (oxidases, digestants, blood-pressure raisers, etc.), 

 or other means the tissue-cells may determine what shall pass by way 

 of the lymph into and out of them. It is surely not wholly a matter of 

 selection by the endothelium, for the blood on one side of the "mem- 

 brane" and the tissues on the other doubtless in a large measure direct 

 the flow of lymph outward and inward. It is easy, however, to carry this 

 "vitalistic" principle of lymph-formation too far and to leave out too 

 completely the hydraulics of these lymph-movements. Some in this 

 way have tried to convince their readers that lymph is not "transuded 

 plasma" from the blood-capillaries slightly altered by the katabolism 

 and needs of the tissue it goes to and comes from, but, rather that it is 

 practically sewage from the tissues, bound outward into the excreting 

 blood; from this aspect the blood-flow supplies the tissues with their food. 

 In reality, it is only as a rude simile and then wholly in a mechanical sense 

 and for one functional phase only that the lymph may be said to be like 

 the sewerage-plant of a town. Rather is it like a combination, impossible 

 outside an organism, of the water-supply and sewerage-system of a 

 place, and then only in a mechanical, not at all in a chemical, sense. 



Into the formation of lymph at least three processes enter, one being 

 physical in nature, one chemical, and one probably having elements 

 of both these others. The physical conditions of lymph-production are 

 the pressure and the flow in the blood-capillaries; the chemical condition 

 is the osmotic pressure of the lymph itself, dependent on molecular and 

 ionic concentration; the mixed condition is the permeability of the blood- 

 and lymph-capillary walls. The last is a matter of the constitution of 

 protoplasm not at present definable, but without doubt chemophysical 



