INTRODUCTION 6 



it was asked how those structures "knew" when the content was 

 the same as usual, and ''knew" what to do when it differed. 

 Further it was observed that the regularity of water content is 

 related to systematic modifications in the exchanges through those 

 or other structures, in accordance with the actual contents prevail- 

 ing. To certain contents the body is indifferent or acquiescent ; in 

 the presence of unusual amounts of the same components there is 

 violent action that usually succeeds in changing them. Moreover, 

 however satisfying it may be to know that kidneys or alimentary 

 tracts are pathways that care for the exchanges of water, it is soon 

 realized that many organisms exist without those organs, and yet 

 probably every organism has means of correcting unusual water 

 contents. Further still, an organ to remove excesses and another 

 organ to make up deficits serve the body only so long as each grades 

 its activities to whatever disturbance of content prevails, and only 

 so long as the two organs are related to one another, in agreement 

 as to what water content will be accepted by both. 



Is an organism actually exposed to unlimited loss, or to 

 unlimited gain ? This topic concerns structural, mechanical, chemi- 

 cal and other features that somehow protect the body from suffer- 

 ing rapid changes, in the mode of existence peculiar to its species. 



But is water content entirely a matter of faster and slower 

 exchanges? Does its constancy not depend on availability of 

 water in the environment? Water content is usually part of a 

 stationary state that requires a source and a sink; these are en- 

 vironmental features toward which the behavior of the organism 

 is frequently found to be appropriate. 



Biologists in all ages have visualized one or another of these 

 questions. Taken together, their views (§ 157) form a valuable 

 background to an intensive treatment of self-maintenances. As 

 long as 2400 years ago, Alcmaeon stated that "The preservation 

 of health consists in an accurate adjustment of forces." This is 

 the oldest known generalization of physiology. It represents the 

 vague realization that the normal state of the organism is main- 

 tained by specific processes which resist change. And today it 

 persists as a fruitful generalization that can be sharply delineated. 

 By methods that were scarcely tried by my predecessors I can hope 

 to fit together observations that help in comprehending the manner 

 in which constancies of many properties are maintained. I seek to 

 describe the operating characteristics of these maintenances. 



