INTRINSIC ACTIVITY OF SECRETING CELLS 257 



saturation point has been passed. Thus, although at two atmo- 

 spheres of oxygen pressure the Amount of oxygen dissolved in cor- 

 puscles, plasma, and tissue cells is not very appreciably higher than 

 when the oxygen pressure is about 100 mms., yet the activities of 

 the cells become affected and the animal dies in convulsions. So 

 although the uptake of potassium ion by the cell may not be appreci- 

 ably affected quantitatively when the concentration in the plasma is 

 increased compared to the uptake at a lower concentration, yet the 

 physiological action of the small additional amount upon the cell 

 may be enormous. 



It must be remembered that just as in the case of oxyhsemo- 

 globin there is no absolutely definite pressure which can be spoken 

 of as the dissociation point, but rather a definite range of pressure, 

 throughout which association of the oxygen and hemoglobin 

 occurs, so in the case of other dissolved constituents (ions, organic 

 crystalloids, and anaesthetics) and the tissue cells, there is not a sharp 

 point, but a range of association with increasing pressure, and the 

 curve of osmotic pressure and association varies with each dissolved 

 constituent and each type of tissue cell. 



In the case of every active drug, and every active constituent 

 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 proto- 

 plasmic 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 protein substance fully combined 

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



17 



