126 



PHYSIOLOGICAL REGULATIONS 



<^43. Rat 



Excesses of water administered by stomach to rats (Epemys) 

 are not fully returned in urine. Figure 75, confirmed by new tests, 

 shows that about one-third of the water ingested, though absorbed, 

 disappears in evaporation before the original body weight is re- 

 stored. If this one-third is included in the water excess, which 

 thus represents the total load instead of the sensible load, then the 

 relation (D) between rate and load is like that found in the dog 

 (fig. 8 in § 7). Here is evidence that total water loss is more regu- 

 larly concerned in recoveries from excesses of content than is 

 urinary loss alone. The clearness of this evidence depends upon 



o 



a 



DC 



Water Load 



Fig. 75. Eate of urinary water output (% of Bo/hour) in relation to mean water 

 load (% of Bo) established by a single administration of water by stomach in rats. 

 Each point represents a successive period of 0.33 hour. D, total load in 18 individuals 

 of Heller and Smirk ('32a, p. 15), and as modified (BD) when absorption is measured 

 in 47 sacrificed individuals. C, sensible load in the same 18 individuals, and as modified 

 by absorption (BC). l = half of load returned. 



study of this species in which evaporative losses are faster (rela- 

 tive to body weight) than in dog or man. Since absorption (B) is 

 complete before diuresis reaches its maximal rate, no account need 

 be taken of the rates of absorption as measured, in describing 

 recoveries over periods later than 1 hour after ingestion. 



Recovery from a deficit of water is a gradual process in rats 

 (fig. 76). No matter how the water is presented, only the first 

 minute or two of drinking is rapid and continuous. Here is a spe- 

 cies that characteristically drinks at rates intermediate between 

 dog's and man's ; still, no special relation of the rate of drinking to 



