36 



PHYSIOLOGICAL REGULATIONS 



ments ' ' of water in heat loss, in solute excretion, in swallowing of 

 food, in water content of food, and in others. It is possible to 

 inquire whether any one factor is likely to limit the smallness of 

 turnover, to prevent the animal from getting along without water. 

 So far as I can see, no one factor is limiting, and several of them 

 insure that some water exchange will always go on. 



In another direction, it is possible to inquire what properties of 

 the dog distinguish its water losses from those of a sack of salt 

 solution. The sack might be of rubber, allowing no exudation, or 

 of any one of many materials that allow water to pass at diverse 

 rates. Eimer ( '26) found that the body surface of the dog lost by 

 evaporation about 0.017% of Bo/hour to air of 23° to 27° C. and 



4 - 



-4 -2 *£ ^4 *b 



Total Woier Load 



Fig. 16. Rate of net water exchange (% of Bo/hour) in relation to total water load 

 (% of Bo). Net exchanges are either total gains minus total losses or total losses minus 

 total gains. These curves are derived from figure 13, and represent the rates of recovery 

 within the first 1.0 hour of recovery. 



33 to 62 per cent relative humidity (43 determinations on 4 indi- 

 viduals). This is about 5 gm. per square meter of skin surface an 

 hour (Trolle, '37). Local evaporation from man amounts to about 

 6 gm./m.^ hour (Pinson), while from a water bath kept at 32° C. it 

 varied from 48 gm./m.^ hour when still, to 240 gm./m.^ hour when 

 stirred steadily. Air motion and other conditions promote evapo- 

 ration still further. All of this indicates that the dog's body sur- 

 face does not allow free evaporation, but allows an amount small 

 enough to be consistent with its heat turnover. Modification of 

 evaporative rate, of course, depends chiefly on the respiratory 

 surfaces ; but such modification is found to be unrelated to water 

 load except at extreme excesses. 



If the dog, except the head, be immersed in water, exchanges 



