Chapter XII 



FURTHER CORRELATIVES OF WATER CONTENT 

 AND EXCHANGES 



§ 93. The correlatives of body water that have been examined 

 are only some of the many possible ones. A few other factors in- 

 volved in water exchanges may now be considered. How are they 

 related to maintenance and to equilibrations of water content? 



§ 94. Possible forces 



Very often there is a strong urge to add to the study outlined, 

 the step of inferring an ''underlying" force in each water distri- 

 bution and water exchange. Without the thought that something 

 underlying can be investigated, it is said, there is no incentive to 

 obtain data. An incentive may well be indispensable to investiga- 

 tion; but I find scant evidence that all the forces concerned in an 

 exchange are ever identified. Thus, ''osmotic" pressure may be 

 inferred to be operative in some cases, from relations between con- 

 centration gradients and rates of water exchange. But other sorts 

 of forces, both known and unknown to physicists, may be simul- 

 taneously present, yet described also by the very equations derived 

 on the assumption that only "osmotic" pressure prevails. In 

 brief, forces tending to restore water content may be differentiated 

 in part, and summed to equal the net force present. The categories 

 are labels by which a physiologist thinks about movements of water 

 in terms of another science, and remembers and predicts what 

 happens under specified conditions. Several instances may be 

 cited. 



(1) Forces concerned in water exchanges of many aquatic 

 organisms are supposed to be chiefly differences of osmotic pres- 

 sure between living unit and medium. For eggs of Arhacia, di- 

 verse media affect rates of exchange roughly in proportion to their 

 relative concentrations (fig. 91). A physiologist interested in 

 labelling forces then chooses whether (a) to define osmotic pressure 

 as the total of all forces that are proportional to concentration, or 

 (b) to assert that osmotic pressure probably represents a majority 

 of the forces at work in moving water and regulating its exchanges. 

 In either case what is accomplished is to classify forces according 



255 



