258 PHYSIOLOGICAL REGULATIONS 



imagined to be considerable. The effective osmotic pressure of 

 this common medium might be great toward one kind of cell, less 

 toward another. Possible diversity of properties among cell types 

 thus begins to receive recognition. 



For terrestrial vertebrate individuals, more diverse forces are 

 suggested, as those operative in kidneys and in the neuromuscular 

 apparatus of intake. For cutaneous tissues of frogs, '^ electro- 

 static" forces have been imagined to compete against "osmotic" 

 pressure in intake. Often it is implied that single cells, or worms, 

 or some other category of living units, are incapable of performing 

 exchanges for which mammals use special organs. That view fails 

 to recognize the fact that water is exchanged and adjusted without 

 such organs. 



So there are theories as to the physical types of forces for 

 several kinds of units in which water exchanges have been ob- 

 served. And it is probable that recognized categories can be found 

 to supply suggestions for each of the very many living units that 

 have not been yet observed. In general, it is doubtful whether any 

 one type of force is more prevalent in the water exchanges of all 

 varities of cells than it is among all varieties of emunctories. 

 There are plenty of cells even in dogs that are not protected by an 

 ''internal medium" from extreme changes of concentration about 

 them ; constancy of volume in them may require compensations and 

 adjustments managed as intrinsically as in bacteria. There are 

 also assertions that known varieties of forces taken together are 

 insufficient to explain the phenomena found, and I believe such 

 assertions are just as helpful and as helpless as those to the con- 

 trary. 



In practice, a force is no more ultimate than a rate. Both are 

 equally implicated in regulations, and whatever superiority as a 

 working hypothesis a force might have over a rate, its descriptive 

 value is distinctly inferior. Until it is demonstrated that an 

 "underlying" force is more than a parameter in an equation, is a 

 phenomenon measurable in its own right, the understanding of 

 water exchanges in organisms is not limited to instances in which 

 the nature of a force can be recognized. 



§ 95. Peemeability 



Rates of water exchange are often ascertained under conditions 

 where quantities are combined in dimensions equivalent to "perme- 



