70 



PHYSIOLOGICAL REGULATIONS 



in just this situation ; the total water in the body increases with age 

 but the proportion of water to dry matter decreases (Thomas, '11). 

 Privation of food may accompHsh the opposite (Bothlingk, 1897; 

 Witsch, '26), decreasing the total water and increasing the propor- 

 tion of it in the body. A drink of 0.08 M solution of sodium chlo- 

 ride increases the water content both '" absolutely" and relatively, 

 but one of 3.4 M only absolutely. 



No sharp criterion (other than recovery itself) is alone suffi- 

 cient to distinguish (a) shifts of content at balance (Wo) from (b) 

 changes of water content (AW) without shifts of the Wo to which 

 it will return when allowed. Possible criteria would be the demon- 



+4 



.T+2 - 



o 

 CQ 



c 

 a 



E -2 - 











10 



20 



30 



Days 



Fig. 43. Sequence of body weights upon 27 successive days. The dog received 

 constant food once daily; water was continuously available. New data. 



stration that rates of exchange at some three or four diverse water 

 ■contents have changed (fig. 42). Biochemical analyses that could 

 determine the water content are at present much less accurate than 

 the measurements of physiological recoveries that ensue. Over 

 periods of time exceeding a few days, or in inconstant conditions 

 'of food intake, it is customary to credit changes of body weight {i.e., 

 of water content) to shifts of the zero water load {e.g., see fig. 43). 

 In general, wherever prompt adjustment of water content to zero 

 load does not occur when conditions are judged proper for recov- 

 ery, it is tentatively supposed that Wo has suffered change. 



One further step is suggested. When it becomes possible to 

 select some criterion by which the water content of the dog is mea- 

 sured independently of the water exchanges, then diverse modifi- 



