Cellular Aspects of Body Electrolytes and Water 27 



the high concentration of chloride in the cerebrospinal fluid, 

 which may be taken as a measure of this hyperosmolarity, 

 appears at an early stage in development — at about 40 days 

 in fact. It may be that the positive pressure of the cerebro- 

 spinal fluid depends for its maintenance on a difference of 

 osmotic pressure between it and the blood. 



The factors determining the water and electrolyte contents 

 of connective tissue are probably simple, although they have 

 not been studied in great detail. If a piece of collagen, or 

 collagen plus mucoid, is placed in a saline medium, equivalent 

 to extracellular fluid, we may expect a Gibbs-Donnan equili- 

 brium to be established between this and the medium by 



Na+ Cl- 



Na+ Coll 



Na+ CI 



Na+ CI- 



Fig. 5. Illustrating Gibbs-Donnan equilibrium 

 between collagen and extracellular fluid. In 

 this case there is no membrane separating 

 the two, the collagenous gel being a separate 

 phase. 



virtue of the acidic nature of the protein and mucoid. The 

 situation might therefore be as in Fig. 5, i.e. essentially 

 similar to that obtaining with plasma separated by a mem- 

 brane from extracellular fluid. There is no membrane separat- 

 ing the two, however, and separation is maintained because 

 of a phase difference, the collagen-mucoid system being a gel, 

 the extracellular fluid a liquid. Chemical analysis of con- 

 nective tissue shows that there is, indeed, a Gibbs-Donnan 

 distribution of ions between it and plasma and therefore, 

 presumably, between it and extracellular fluid, the concen- 

 tration of chloride being less, and that of sodium greater, in 

 the connective tissue. There is, in consequence, a tendency 

 for water to pass into the connective tissue phase, the salts 



