194 PHYSIOLOGICAL, EEGULATIONS 



both kinds of loads, and within that number fall all the instances 

 known. 



An additional type of equilibration diagram that might be ex- 

 pected to occur in some species is one with a broad range of water 

 contents within which no rapid adjustments occur. There would 

 then be considerable leeway between unallowed excess and unal- 

 lowed deficit ; a stretch of water contents to which the organism is 

 indifferent. Helix seems to correspond to this type, though figure 

 83 is too coarse to show it; according to Howes and Wells ('34) 

 and previous investigators it eats every day but ingests water only 

 every several days. Hence large rhythmic fluctuations of water 

 content occur, averaging at daily intervals ± 8% of Bo (table 12). 

 Variations of less than ± 0.5% of Bo are often found in dog, man, 

 and rabbit, giving rise to neither diuresis nor drinking. Larger 

 excesses usually fail of complete excretion by small margins. The 

 camel can store much "unrequired" water in its peculiar stomach, 

 but the amount in relation to water load and water exchanges has 

 not been measured (Howell and Gersh, '35). 



All this relates to the angles a and 3 in figure 110. These angles 

 measure the change of rate (of gain and of loss respectively) with 

 increasing ± AW, at the most used parts of the range of possible 

 water contents. Perhaps they measure an irritability of the organ- 

 ism, being the increment of load that calls forth an increment in 

 rate of exchange (response). 



I do not conclude that there is any quality very unique or pecu- 

 liar about the equilibration of water in living organisms. I do 

 conclude that many, and probably all, organisms, organs, and cells 

 have machinery for getting rid of excesses of water or preventing 

 the accumulation of excesses, or both. In fact, with Bernard (see 

 § 4) I doubt whether any organism exists without adjusting its 

 content of water. But some adjust rapidly, and some slowly. 

 Non-living systems such as gelatin blocks, water tanks, city reser- 

 voirs, and many others, have equilibrations of water, and would 

 yield diagrams for water exchanges with some of the properties 

 already outlined. Possibly there are constant and significant dif- 

 ferences in such systems as contrasted with living ones, so that, for 

 instance, in them recovery does not go to completion after a variety 

 of displacements of water balance. Such distinctions might be 

 valuable in comprehending the properties of both the living and 

 the non-living systems. 



