. WE t IIANISM OF ABSORPTION. 307 
the tissues. If, however, we consider a system in which the inner 
tube is connected al various points in its circumference to the outer 
tube by strands of fibres, it is apparent thai 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 tilnes 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 now. In the former case a back filtration would lie 
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 amoimt 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 lie found in a property 
on which much stress was laid by the older physiologists, and which 
they termed the high endosroptic 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 Arch.f. Physiol., Leipzig, 1895, S. 281. 
