262 PHYSIOLOGICAL EEGULATIONS 



In the end, it is rather arbitrary to segregate under the term 

 permeability certain classes of water exchange, namely those in 

 which the investigator thinks he can see and estimate the force that 

 operates and the surface of exchange. Where thickness of bound- 

 ary is also known, separate classes would be studied under the 

 term diffusion. Where the area of exchanging surface is unknown 

 or invisible, another related coefficient may be used, i = SW/(At X 

 AP ) . Avoiding the factor of area, this ' ' coefficient of osmotic flow ' ' 

 {i = hy( S) also avoids complications connected with the presence 

 or absence of convection behind the boundary, such as by the blood 

 stream; for the coefficient may include any such factors (Bohr, '09, 

 p. 251). Where the pressure under which exchanges occur is un- 

 known, it is sufficient to measure the rate of exchange, Rw = AW/ At ; 

 hence Rw = /^ X S X AP. 



It is apparent that the arranging and ordering of experimen- 

 tally measured values of water exchanges reduces itself to ascer- 

 taining other possible quantities ivith which to correlate the 

 measurements of rates. Some of these may be combined, reducing 

 several variables to one; occasionally several variables yield a 

 constant, furnishing a useful and economical description. 



Whether or not it is worthwhile to compare water exchanges 

 through alimentary tracts, kidneys, and antennal glands with those 

 through surface membranes of supposedly less differentiated sorts, 

 is a matter of opinion. The ''pressures" involved in securing, 

 swallowing, and absorbing a drink of water in a mammal would 

 require precise definition, and for the present they might be re- 

 garded as incommensurate with the pressures under which fluid 

 crosses the walls of blood capillaries. Were someone to label them 

 ''psychic pressures," physiologists would try to avoid measuring 

 water intakes for many decades, I suppose. Yet a water deficit, 

 particularly if considered as a concentration increment, is propor- 

 tional to something that has the dimensions of pressure and might 

 be put into the equation by which the coefficient i is computed. 



In brief, coefficients of permeability and of osmotic flow combine 

 the values for rates of water transfer with factors of pressure 

 gradient. They may advisedly be defined so as to be independent 

 of any particular mode of transport such as diffusion, of any par- 

 ticular force such as osmotic pressure, and of any implication to 

 constancy. While those coefficients may be compared for only a 

 few organisms and parts under specified conditions, rates of water 



