MECHANISM OF ABSORPTION. 307 



the tissues. If, however, we consider a system in which the inner 

 tube is connected at various points in its circumference to the outer 

 tube by strands of fibres, it is apparent that a rise of pressure in the 

 space surrounding the inner tube will only serve to extend this tube 

 still further. No collapse will take place, but a back filtration will be 

 possible. If we cut sections of injected connective tissues, we find 

 that the capillaries are bound to surrounding parts by radiating fibres 

 which might possibly prevent their collapse under high extravascular 

 pressure. In the larger veins, on the other hand, the arrangement of 

 the fibres of the adventitia is circular and not radial, so that a high 

 extravascular pressure would apparently cause collapse of the veins. 

 From these anatomical facts one would conclude that a backward filtra- 

 tion is possible, provided that the extravascular pressure be raised in the 

 region of the capillaries. If, however, the pressure be freely propagated 

 through the tissues so as to affect the larger veins draining them, we 

 shall have collapse of the veins and increased oedema. Here, as in so 

 many other cases, we cannot get a decisive answer to our physiological 

 questions by purely anatomical investigation, but must have recourse to 

 physiological experiment. 



The question that we have immediately to decide is, whether an 

 increased tissue tension augments or leaves unaltered the flow of blood 

 through the tissues, or whether it causes venous collapse and so 

 diminishes the flow. In the former case a back filtration would be 

 possible, and in the latter case impossible. I have investigated this 

 point in various regions of the body, e.g. the connective tissues of the 

 leg, the tongue as a type of muscular tissue, and the submaxillary gland 

 as a type of glandular tissue. In all these cases I have found that a rise 

 of tissue tension above the pressure in the veins causes collapse of these 

 veins, a rise of capillary pressure, and a diminished flow of blood through 

 the part. In these regions of the body, therefore, absorption of lymph 

 by a backward filtration is impossible. 



Imbibition. Hamburger, 1 finding that serum and isotonic fluids are 

 absorbed from the peritoneal cavities of animals that have been dead some 

 hours, concludes that the life of the endothelial cell can have nothing to do 

 with the process, and ascribes the absorption to processes of capillary and 

 molecular imbibition, so that the absorption of fluids would be analogous 

 to the taking up of fluids and gases by animal charcoal. Though these 

 factors probably co-operate to a certain extent in the distribution of the 

 fluid through the tissues surrounding the serous cavities, it is evident 

 that they would be much more pronounced in dying and disintegrating 

 tissues, and could with difficulty explain the taking up of fluids by the 

 blood vessels. They would certainly not explain the wonderful balance 

 which exists between the intracapillary pressure and the amount of 

 fluid transuded from or absorbed by the blood vessels. What, then, is 

 the explanation of this absorption ? 



Osmosis. The explanation is, I believe, to be found in a property 

 on which much stress was laid by the older physiologists, and which 

 they termed the high endosmotic equivalent of albumin. It must 

 be remembered that the older physiologists used animal membranes 

 in their experiments on osmotic interchanges. These membranes permit 

 the passage of water and salts, but hinder the passage of coagulable 

 proteid. The application of semipermeable membranes to the measure- 



1 ArcJi.f. PhysioL, Leipzig, 1895, S. 281. 



