COERELATIVES OF WATER CONTENT 207 



other types of variables. How these many correlatives vary to- 

 gether will be shown by a study in which ± AW (water load) is 

 kept as one variable throughout, letting one after another of the 

 many physiological quantities that are measured in relation to it, 

 come under review. Each of them is a part of the complex that 

 goes with water load. The study will be designed to show whether 

 anything that changes with water load inside the dog furnishes a 

 key to understanding what prompts recovery in the body as a 

 whole. 



Ideally, the water load might be of one type in all divisions of 

 the study. Experiments would be set up with dogs deprived of 

 water as defined in § 13 and with dogs administered water by 

 stomach as defined in § 12. Then, within the limits of the station- 

 ary state of water load, diverse analyses and measurements would 

 be made. No such ideal program has been carried through; and 

 not from the difficulty of planning it with singleness of purpose, 

 but from the realization that a strictly steady state with respect to 

 all measurable components {e.g., chloride content of body, heat 

 content of body) does not exist. Practical difiiculties of other sorts 

 creep in, and for the present it seems sufficient to indicate, by means 

 of the partial data from diverse sources that are available, what 

 relations are known to exist. 



First, within one species (dog) several types of changes in 

 water content will be considered. Those correlatives that are 

 peculiar to one type will be detected, and hasty generalizations as 

 to what characterizes all water loads may be avoided. 



§ 78. Volumes of parts 



It is not hard to suppose that endless changes occur in diverse 

 parts of a dog with each load of water. Point is given to the search 

 for those changes as soon as it is asked: Where is the excess water 

 deposited, or whence is the deficit withdrawn? Are there depots 

 for water; what does a reserve of water look like? And, are some 

 tissues especially sensitive to water loads'? Evidently volumes of 

 tissues have to be measured, and not in relation to the tissue's own 

 exchanges (as in §58) but in relation to the whole body's water 

 content. 



At least three general procedures are suitable for measure- 

 ment of increments of volume in any one portion of the dog's body. 

 {a) The difference of net weights or volumes of parts secured at 



