264 PHYSIOLOGICAL, REGULATIONS 



from it. And I want to know whether equilibration of water 

 changes with age independently of body size. 



If body weight had been disregarded, contents and exchanges 

 of water would have been measured in grams or milliliters. No 

 change in the forms or shapes of the graphs representing a single 

 species would be introduced with the new numerical scales, how- 

 ever, for load (± AW) was in every instance correlated with time 

 or with rate (SW/At). In the latter case, increment of water ap- 

 peared in both coordinates, and hence any changes of scales would 

 be proportional in the two coordinates. In many data more than 

 one individual of a species was represented ; this would have been 

 inadvisable when load was measured in grams if the individuals 

 differed in body weight by more than perhaps 10 per cent. Hence, 

 most of the data would have been separate for each body size. 



Another measure of size, such as a length, skin surface area, 

 kidney weight, blood volume, or alimentary tract area, might have 

 been substituted. Any or all of these are, of course, equally ad- 

 missible. Some cannot be measured during life; others can be 

 measured but with much less accuracy than body weight. Very 

 often the new measure may be inferred from body weight through 

 prepared correlations; this is the case with skin surface area, a 

 measure of size that has enjoyed vogue in recent physiology. Is 

 area especially related to rates of water exchange? Equations and 

 constants are already available, in most species considered here, 

 for transforming body weights to surface areas. Surfaces are 

 roughly power functions of body weights; and many of the ex- 

 ponents, found empirically, approximate the value 2/3. Inaccu- 

 racies inherent in those correlations are avoided by relating water 

 exchanges directly to logarithms of weight; and then the impli- 

 cation is not so strong that surface area or length or weight itself 

 has any particular connection, except by convenience of thought 

 and availability of data. 



(1) Within the species. Relations between rates of water turn- 

 over and body weight are established in two species of mammals. 

 In units of centimeter, gram and hour, the young rat shows rates 

 of ingestion of preformed water (in turnover) such that (fig. 

 133) R = 0.041B''-"°-°-'^ ; while believed surface area (Lee, '29) 

 S = 12.5B°-'°. When the equations are combined, R = 0.0033S ; or, 

 in one hour 1/300 milliliter of water is ingested for each square 

 centimeter of body surface. 



