76 PHYSIOLOGICAL KEGULATIONS 



every hour, but waits longer intervals to raise the body's water 

 content. 



A special study was accordingly made (fig. 46) to find how much 

 and how frequently dogs drink, under four sets of conditions at one 

 particular time of day. (l) With water continuously available, 

 but without food, dogs drink within 2 hours only in one of ten tests 

 (E). The single amount drunk is somewhat less than the inter- 

 vening deficit of body weight. (2) With food, water is taken by 

 dogs once to four times after each meal (A, B, C,) even though a 

 meal of dry food be eaten every hour (b). (5) If water is not 

 available for 1 or 2 hours, it is not drunk if offered at the close of 

 either period. At the close of 3 hours (not shown), it is drunk in 

 half the tests, and then in amounts equal to about half the deficit. 

 (4) If the dog be warmed for 1 hour, thus increasing the deficit 

 of water, water is drunk as soon as offered in every test but one, 

 whether offered during the heating (D), immediately afterward 

 (G), or 0.5 hour afterward. 



From this I conclude that a dog does not sip water at short 

 intervals. Most water is taken, as is well known, shortly after 

 food is eaten. But in the absence of feeding or heating, water is 

 likely to be drunk about every 4 hours, in amounts less than suffi- 

 cient to restore the body weight. However, the lapse of time is 

 probably less closely correlated with drinking than the lapse of 

 body weight is, for when deficit is hastened by heating, drinking 

 occurs as soon as - 0.5% of Bq has been reached. That is the great- 

 est change of water content that a dog usually allows without doing 

 something to remove it. 



Whether the fluctuation of water content represents a load or 

 a shift of content at balance, cannot be entirely decided. When a 

 dog stands hour after hour in a stall, losing water by evaporative 

 and urinary paths, leaving water untouched, I may conclude that 

 either (a) the dog is running into negative loads, and the sensitivity 

 of the processes leading to water ingestion is too low to act, or (b) 

 the dog is staying in water balance, water not being required to 

 replace that lost until food has also been taken or until the clock 

 gets around to some other hour. On the latter criterion the usual 

 losses of water are eliminations of excesses that arise as metabo- 

 lism proceeds, and the sensitivity of the responses by intake is pos- 

 sibly as great as of those by output. On the former criterion of 

 water content the organism sacrifices water to the benefit of excre- 



