278 
DIFFUSION, OSMOSIS, AND FILTRATION. 
in the intestine purge (sodie sulphate), while others do not (sodic 
chloride), may in the end find its explanation in a permeability of the 
membrane by the latter, but not by the former. A universal 
" physiological salt solution," then, if by such a term is meant a salt 
solution in which tissues neither lose nor take up water, and the dis- 
solved substance of which does not enter the cells, is not a possibility ; 
each tissue must in fact have its own " normal solution," 1 and this may 
possibly be in some cases a solution of some other substance than a 
salt. 
The effective osmotic pressure, therefore, exerted against membranes 
such as those in the body which are, as a rule, partially permeable to 
dissolved substances, is far below that measured by a semipermeable 
membrane, and freezing-point determinations of osmotic pressures 
(determinations which give a gauge of the full osmotic pressure as it 
would be exerted against a semipermeable membrane), are of but 
orienting value to the physiologist, except in cases where the per- 
meability of the membrane to the substance in solution is known. 
The following table from Pfeffer 2 is illustrative of the diminution in 
the estimate of the full osmotic pressure caused by substituting a 
permeable membrane (bladder or parchment paper) for copper ferrocy- 
anide, and it is evident that the effect is far more marked in the case of 
the crystalloids (saltpetre and sugar) than in that of the colloid (gum). 
Six per cent. Solution of 
Parchment Paper. 
Bladder. 
Copper Ferrocyanide. 
Gum arabic . 
17-9 
13-2 
25-9 
Cane sugar 
29-0 
14-5 
2877 
Saltpetre 
20 "4 
8-9 
700-0 
(not directly estimated.) 
The pressures are in ems. of mercury. 
The conditions, then, for the interchange of water and the con- 
stituents of solutions through membranes in the body, are evidently 
exceedingly complex, and it is at present practically impossible to assess 
the value of all the factors. Broadly stated, the following factors are 
concerned : — 
1. The quantitative composition of the solutions separated by the 
membrane, and consequently the partial osmotic pressure exerted by the 
several constituents. 
2. The coefficients of diffusion of the various constituents. 
3. The permeability of the membrane in its physiological condition 
to the constituents. 
4. The circumstances affecting the relative concentrations of a 
constituent on the two sides of the membrane with time, e.g. circulation 
and stirring. 
5. The hydrostatic pressure on the two sides of the membrane. 
G. The temperature. 
The partial osmotic pressure of the constituents of a solution is 
obtainable from a quantitative analysis, if the molecular weight and 
1 Kneppe, Arch. f. ,1. ges. Physiol., Bonn, 1S97, Bd. lxv. S. 492. 
- " Osmotische Untersuch.," Leipzig, 1S77, S. 73. 
