CORRELATIVES OF WATER CONTENT IN OTHER SPECIES 245 



With respect to water deficit, whimsical choice has emphasized 

 the dryness of the mouth. It is said (Cannon, '32, p. 62) that a 

 sensation of ''thirst is due to local dryness of the mouth" and acts 

 ''as automatic stimuli to make certain that the reserves of water 

 are maintained." No method of measuring the sensations as such 

 has been devised, nor of evaluating the dryness. The salivary flow 

 to the mouth has been measured ; since it does not alone control the 

 dryness of the mouth, it itself is not regarded as a regulator. The 

 theory that a single governor exists for a physiological activity 

 does not appear now to be substantiated. Moreover, there is no 

 means of recognizing a regulator even if examined. A specific vol- 

 ley of nerve impulses or an isolatable extract are possible links in 

 a chain, or elements in a complex. Repeatedly it is found that 

 many factors vary simultaneously; each one is as central as any 

 other, and only by convenience of thought is one exalted above 

 another. Hence the emphasis may be put on the interrelations of 

 the diverse variables and the degrees to which the correlations hold 

 in repeated tests, in diverse individuals, in diverse species, and in 

 diverse types of water increment. Thus characterizations and 

 mutual relations are established, all with respect to the one vari- 

 able, water content. 



The foregoing sorts of data should make it possible to design 

 methods for identifying diverse states of water content in man. 

 One task is to differentiate various types of ± AW, a second task 

 is to choose indices by which to estimate the degree of increment 

 or deviation from water balance, a third is to differentiate control 

 states from pathological states. The serious lack of reproducible 

 measurements makes these tasks at present more virtual than real. 



(1) In spite of all the "dehydrations" that have been studied, 

 and the liters of solutions that have been swallowed, it is at present 

 difficult to identify the specific procedure of water loading, from 

 measurements of compositions and metabolisms alone. The clear- 

 est case is to find whether or not solutes (chloride) were added or 

 subtracted along with water, by analyses of their proportions in 

 urine and in plasma. 



(2) Suitable indices of increment in water content are incre- 

 ments in dilutions of urine and of plasma, rates of salivary flow, 

 and augmentations of urea clearance. Dryness of lips or edema 

 of skin are qualitative only. 



(5) Increments in volumes and in dilutions, especially of plasma 



