2 8o DIFFUSION, OSMOSIS, AND FILTRATION. 



noted, however, that animal membranes are known in which the 

 resistance to the passage of fluid is quite different in opposite directions. 

 The most familiar example is the shell membrane of the egg, which 

 permits nitration far more easily from within outwards than in the 

 reverse direction, 1 and the same is true of the skin of the frog. 2 



FILTRATION. 



By nitration is meant the passage of fluid through a membrane, as a 

 result of a difference of hydrostatic pressure on the two sides. To filter 

 water from a solution across a membrane into another solution, the 

 difference between the hydrostatic pressures on the two sides must 

 exceed the difference between the osmose pressures of the solutions, in 

 the case where the higher osmotic and hydrostatic pressures are on the 

 same side. If a porous pot bearing a semipermeable membrane is 

 filled with a solution and immersed in the pure solvent, the pressure 

 necessary to produce filtration of the solvent from the solution is one 

 just exceeding the full osmotic pressure of the solution ; but where we 

 deal with permeable membranes, as those in the body, the necessary 

 pressure is far less, because the difference of osmotic pressure on the 

 two sides of the membrane is reduced by the diffusion of some of the 

 dissolved substance. 



Experiments on filtration through animal membranes appear to have 

 given very contradictory results, which seems to be due to the fact that 

 not only does continued pressure upon such membranes vary their 

 permeability, but a certain amount of " recovery " takes place in the 

 intervals between use ; hence the conditions of the membranes have been 

 by no means uniform in the experiments of different observers. 



If the passage of fluid through an animal membrane is by more or 

 less tortuous paths, and if the walls of these are more or less elastic, it 

 is obvious, not only that by continued pressure must the resistance to 

 filtration rise, but that on removal of the pressure a slow " recovery " will 

 take place. Deformation of the membrane, if simply tied over a tube, 

 must also, unless it is properly supported, tend to distort channels and 

 so increase resistance to filtration. To these sources of difference in the 

 experiments must be added, previous drying of the membrane or not, 

 the condition of imbibition of the fibrous tissue, and temperature 

 variations. All these sources of differences are, moreover, accentuated 

 by the fact that most of the experiments have been made with thick 

 membranes, such as pericardium, bladder, intestine, and ureter. 



Nearly all who have studied filtration have found that at constant 

 pressure the amount of the filtrate falls off with time, 3 but, as pointed 

 out by Tigerstedt and Santesson, 4 this falling off is far more rapid in the 

 earlier hours of an experiment than later, so that it is advisable in all 

 such experiments on filtration to expose the membrane to pressure for 



1 Meckel quoted by Ranke, " Physiologie des Menschen," 1872, S. 122. 



2 Cima, Mem. d. Accad. di Torino, 1853, vol. xiii. ; Reid, Journ. PhysioL, Cambridge 

 and London, 1890, vol. xi. p. 312. 



3 Liebig, " Untersuch. ii. einige Ursachen der Saftebewegungim thierischen Organismus, " 

 Braunschweig, 1848, S. 7 ; Beitr. z. Anat. u. PhysioL (Eckhard), Giessen, 1858, Bd. i. S. 

 95 ; Runeberg, Arch. d. Heilk., Leipzig, 1876, Bd. xviii. S. 58 ; Gottwalt, Ztsclir. f. physiol. 

 Chem., Strassburg, 1880, Bd. iv. S. 423 ; v. Regeczy, Arch. f. d. ges. PhysioL, Bonn, 1883 

 Bd. xxx. S. 544. 



* Bijhang. till k. Svens. Vet.-Akad., Stockholm, 1886, Bd. xi., No. 2. 



