12 PHYSIOLOGICAL REGULATIONS 



(1) Rate of exchange (R), e.g., Total gain of energy, of water 



(2) Species or Living Unit (U), e.g., Dog, erythrocyte 



(3) Path of exchange (p), e.g.. Urinary, radiative 



(4) Tissue (portion) studied (s), e.g., Blood, whole body, cell 



(5) Component (J), e.g.. Water, heat, pressure 



(5a) Type of displacement (f), e.g.. Privation, injection 

 (5b) Quantity of one component (C), e.g., Excess, deficit 



(6) Temporal parameter (t), e.g.. Time, duration 



The number of kinds of correlation among 6 variables is 57, or 

 taken two at a time is (6 X 5)/(l X 2) or 15. The number of spe- 

 cific data available in each class to be correlated is semi-infinite. 



The procedure chosen is, at first, to keep the component (J) 

 constant, then the species or individual or living unit (U) con- 

 stant. Within the study of one component, the type or means of 

 disturbance (f) influencing the component is recognized; later the 

 tissue or other portion measured (s) is differentiated. This re- 

 duces any one set of data to three variables (J, p and t) to be dealt 

 with; p often can also be selected and held constant through one 

 series of data. The other three are entered in separate correla- 

 tions, until summaries calling for regrouping are required. A 

 more complete view of the course pursued may be gained after 

 part of the treatment has been covered (§ 101 and § 152). 



Not all the possible combinations of variables are presented. 

 To do so would be a completion but also a tedium. Those omitted 

 are not less important to the organism, so far as anyone can judge ; 

 but are of three classes: those for which suitable data were not 

 obtained, those unfamiliar to me, and those in which the correla- 

 tion did not seem to me to yield illuminating relations. Limita- 

 tions are imposed also by the number of variables that can be con- 

 veniently and profitably handled at one time. Variables, in fact, 

 may be numerous and unknown, but the ones recognized are chosen 

 for their reproducibility, statistical significance, and apparent in- 

 terrelations. Only by continual classification and limitation can 

 the investigation be kept, at each point in its progress, within 

 comprehensible limits. Some of the studies here presented may 

 appear to be exhaustive. But I am the last one to consider them 

 so, for hosts of lacunae and possible extensions have come into 

 view. It is conceivable that physiology of the future will be still 

 more quantitative, more detailed, and more interested in interrela- 

 tions and variabilities. 



