388 PHYSIOLOGICAL EEGULATIONS 



rate of emptying, and that limitation necessitates a separate term 

 for rate of gain. 



Accumulation rate is such a term. Ordinarily it is related to 

 body mass and has the dimensions L^L"^T"^, but frequently it too is 

 referred to unit of believed surface of exchange. Occasionally it is 

 restricted to net exchanges, and to movement against a gradient of 

 concentration. 



Invasion coefficient (Bohr, 1899) was devised to express the 

 changes of phase (partitions) involved in intake of oxygen and out- 

 put (evasion) of carbon dioxide. It recognizes the differences of 

 partial pressures existing at two ends of a gradient, and includes 

 ostensibly more processes than diffusion. It is the number of mole- 

 cules passing down the gradient of partial pressure per unit of 

 surface and of time (L^L"^ P'^T'^). Certainly the terms invasion 

 and evasion have emphasized the direction more than any other 

 factor of exchange. 



Permeability coefficient is usually defined so as to apply only to 

 such movements of substance or heat along a gradient of pressure 

 or concentration as are induced by the gradient itself. But it also 

 recognizes the surface area of exchanges (L^L"^ P"^T"^). Some- 

 times the coefficient (7?) and time (T) are lumped together into a 

 ''minute number." Wherever the thickness over which the gradi- 

 ent extends is also measurable, a coefficient of diffusion (Fick, 1855) 

 may be computed (L^L"^ L^ P'^T"^). A coefficient of osmosis may 

 be kept separate to designate the molecular movement of water. 



Velocity quotient has been used here in connection with phe- 

 nomena of excretion. In all but name it has long been current in 

 connection with changes of excitability (time constant), rate of 

 relative growth (increment of size per present size per unit of time) 

 and certain other physiological events (velocity constant). Possi- 

 bly its virtual use is coeval with the expression of biological phe- 

 nomena by exponential equations. It is the change of quantity in 

 a given time per unit of quantity present (L^L'^T"^), or, by exten- 

 sion, any T"\ It may be computed from any of the above para- 

 meters of exchange, and often vice versa. 



Recovery rate has had no previous distinct vogue, though often 

 employed qualitatively. It might be defined as the net rate of 

 exchange relative to the load (L^L"^T"^). While the load is dimin- 

 ishing it is not equivalent to metabolic rate, but relates the ex- 

 change to the load of the component being exchanged, and not to the 



