CORRELATIVES OF WATER CONTENT IN OTHER SPECIES 253 



excesses by hormonal or nervous pathways led to searches for hor- 

 mones in blood, brain, kidneys, and urine. I think it helpful to sup- 

 pose that all the modifications identified are equally significant 

 parts of a many-sided physiological state. 



In a study of physiology in the most general sense, it is useless 

 to compare individual itssues, for none of them is common to all 

 animals. Hence tissue comparisons are automatically limited to 

 mammals, or to vertebrates, or to some one or few phyla. Organs 

 having the same name may not be phylogenetically homologous 

 even within the phylum (as, the several types of kidneys of verte- 

 brates). Tissues of the same name may be compounded of diverse 

 elements in extremely different proportions (as, the skin of verte- 

 brates). Since the same name represents a variety of physiologi- 

 cal as well as morphological units, the significance of numerical 

 comparisons of water in tissues is accordingly limited. 



The local distribution of water excesses and deficits, character- 

 izing various species, concerns not merely the relative dilution of 

 the tissue, but also the proportion of the whole body that it repre- 

 sents. When the absolute increments of water in each tissue are 

 added together, a sum equivalent to the total body load (AW) is 

 obtained. Approximately it is found that the same kinds of tissues 

 take the bulk of increment both in excess and in deficit (table 29). 

 They are blood, muscle, and skin, the most abundant tissues. 

 Among species there are clear differences, muscle playing its 

 largest role in dog, skin in all species. In reality all tissues are, 

 in small part, reservoirs of diverse size and distensibility. The 

 order in which relative dilutions occur are usually (table 29) : skin, 

 blood, muscle, stomach, liver, brain. 



In water shortage does the volume or dilution of the blood 

 plasma (the ''fluid matrix" of cells) change less than the volume 

 or concentration of the body as a whole (Gamble, Underbill) 1 It 

 may be remarked that constancy of medium in which living cells 

 are working can be preserved only at the expense of constancy of 

 cells. I find no evidence that blood has less than its share of water 

 load, either in dog or in man, in frog or in rabbit, and I know of no 

 positive evidence in any other animal. In the frog's water deficits, 

 where the data are adequate, half the tissues lose less than the aver- 

 age, half lose more, and the blood loses most of any. In dog, man, 

 and rabbit, the blood volume is changed as much as or more than 

 the body as a whole. Contrary conclusions might be arrived at by 



