202 PHYSIOLOGICAL REGULATIONS 



most questionable data are those in water privation ; several days, 

 even months, may be occupied to induce the deficit, and food is 

 often refused by desiccated mammals and others, leading to greater 

 deficit of weight but relatively less deficit of water than would 

 occur from lack of water alone. 



Diverse increments of water are tolerated by the various 

 species. More factors than the proportion of water to other sub- 

 stances in the body are at stake. Time is one of these ; with slow 

 desiccation (-40% of Bo in 120 hours) frogs withstood much more 

 loss than with rapid desiccation (- 12% of Bq in 0.7 hour) (Almeida, 

 '26). A thorough study of the numerous factors modifying the 

 survival after deficits and excesses of water would be an extensive 

 investigation. It is noteworthy that dogs are reported to survive 

 the physiologically ''crude" procedure of injecting water intra- 

 venously almost as well as (repeatedly) ingesting water (Falck, 

 1872 ; Rowntree, '23; Chiray et al, '38). 



Tolerated loads might alternatively mean those increments 

 limited by some event other than survival and recovery. Toler- 

 ances according to the criterion of vomiting, or convulsions, or 

 anorexia, or anatomical lesions, or continuance of diverse func- 

 tions, are of equal interest though automatically restricted to cer- 

 tain phyla. The difference between tolerance against loading and 

 tolerance against survival is illustrated by the fact that dogs with 

 extreme contents of water may become unable to excrete it rapidly 

 (Harding and Harris, '30) ; the excretory processes are tempo- 

 rarily depressed, yet after a time the processes may recover and 

 the dogs survive. 



The prevention of loads by various means is a part of the regu- 

 lation of water content to be considered here. A dog often resists 

 the establishment of large excesses by vomiting ; that resistance is 

 of great moment in avoiding loads that might not otherwise be tol- 

 erated. And the dog resists the occurrence of large deficits by 

 having a body surface that minimizes evaporation. In both cases, 

 prevention accomplishes the same sort of stabilization of water 

 content that recovery (compensation) accomplishes; in both the 

 rates of exchange are the tangible evidences of processes concerned. 



§ 76. Summary 



Beyond this chapter the formal study of water in animals will 

 no longer be limited to the four sorts of variables and their quan- 



