152 INTRINSIC ACTIVITY OF SECRETING CELLS 



of the plasma, some such, association must occur as the pressure 

 of such constituent in the plasma rises, and dissociation (with 

 recovery in the case of a drug) take place as the pressure falls. 

 No drug or other substance can be active unless it either enters 

 the cell, and forms some combination with the protoplasm, or 

 else prevents in some manner association and dissociation of a 

 like type in the case of some other important constituent necessary 

 to normal protoplasmic activity. 



The action of different drugs, their rapidity of action and their 

 dosage, will depend on the nature and extent of the association 

 between them and the cell protoplasm. If the saturation point 

 of the drug and protoplasm is attained at a low pressure and with 

 a low amount of drug, then the amount of the drug necessary 

 to produce the full effect will be small, and in all probability the 

 cell will take up but little of the drug, so that to chemical analysis 

 the uptake may appear to be zero, and yet physiological methods 

 of examination may show that the effect is very profound. 



For example, in the case of salts of iron, the saturation pressure 

 must be excessively low, and a proteid substance fully combined 

 with iron contains but a very low percentage of iron, hence the 

 physiological effect of iron may be very large, although the uptake 

 is infinitesimal, and the time required for uptake is large. Thus, 

 in an individual weighing, say, 60 kilograms, the weight of 

 blood would be approximately 4 kilos, that of haemoglobin 

 about 500 grm., and in this the iron would be about 0-4 per 

 cent., or 2 grm. Therefore in a course of iron treatment lasting 

 over some weeks the amount of iron necessary to be taken up in 

 order to produce a marked effect would be so small as to be 

 entirely beyond the bounds of determination under the conditions 

 of experiment. 



Nor does the view of varying permeability of the cell to differ- 

 ent dissolved substances, of high permeability for some and low 

 permeability for others, give any better solution to the real problems 

 of secretion and absorption than that of complete impermeability. 

 For the simple reason that variations in permeability form a passive 

 factor like the variation of a resistance, and hence can at most 

 alter the time relations of the process, and not the end results, 

 and so there can on such a basis be no explanation of the fact that 

 work is done by the secreting cell in the process, as when a con- 

 stituent dissolved substance is secreted at higher concentration 



