384 PHYSIOLOGICAL KEGULATIONS 



is to maintain these endless activities in balance with the endless 

 activities of other organs, and not merely to perform one specified 

 action" (Haldane, '17, p. 85). 



Where paths are still less visibly specialized, as in capillary sur- 

 faces and cell surfaces, it is said that "permeability" controls 

 exchanges. It is often inferred that (a) permeability is fixed, and 

 that (b) one surface limits all components. Neither of these in- 

 ferences seems at present justified by adequate data. Rather, there 

 is evidence that exchange of the one component in deficit or excess 

 is modified without modification of exchange of many other com- 

 ponents. This fact points to equilibration of the same sort as in 

 whole organisms. It means that (a) conditions for exchange 

 (permeability) are not fixed, (b) permeability is not just a limiting 

 factor, (c) permeability is not all one function, and (d) permeabil- 

 ity probably is not vested in an ordinarily pictured monolayer. 



I find no evidence upon which to base a conclusion that dis- 

 posals by storage or by chemical tranformation are faster than or 

 slower than disposals by elimination or by translocation. Even in 

 the same species supposedly identical paths do very dissimilar 

 things. Each component still requires study in its own right ; no 

 rules have been uncovered by which the rates of exchange may be 

 predicted. 



For almost any one of the components that have been investi- 

 gated it is possible to distinguish two or more paths of simultaneous 

 exchange. In those same components, however, only one path 

 varies its rate of exchange with any one load. The impression might 

 be gained that the constant exchange (turnover) is by unavoid- 

 able paths ; whereas the particular path that interferes least with 

 other components is the sole department of adjustment, suiting its 

 rate of specific activity to the special task before it. In the case of 

 glucose (dog) three paths of adjustment were distinguished, each 

 of which varied its rates of exchange over restricted ranges of load, 

 and no two were alike. Or, three disposals came into operation in 

 diverse proportions ; synthesis, oxidation, and excretion. There is 

 probable utility to the organism in this precise arrangement of 

 functions and in the exact overlapping by which they share with 

 one another and reinforce one another. The competition among 

 the paths of an organism is a quantitative ordering of activities so 

 that excretion does not seize that which synthesis will preserve, and 

 vice versa. 



