78 INTIMATE ASSOCIATIONS OF INORGANIC IONS 



if calcium be too abundant we have the heart stopping in 

 the systole of calcium rigor. 



Now the affinities of certain kinds of protoplasm for certain 

 ions are quite different from those of other kinds of protoplasm 

 for them. Thus the red corpuscles fix potassium and iron, the 

 brain, phosphorus ; the muscles, potassium ; the bones and teeth, 

 calcium and fluorine ; the thyroid gland, iodine ; and the fluids 

 of the body chiefly sodium. The thyroid gland can, moreover, 

 fix more iodine per unit of tissue than can any other tissue. 

 Chemically speaking, therefore, protoplasm in different situa- 

 tions is chemically different; the protoplasm of the brain has 

 not the same atomic affinities as that of muscle or bone or 

 thyroid gland. The tissues are, however, supplied by lymph 

 of practically uniform composition, so that these chemical 

 differences have been said to be due to "selective affinity." IS^ow 

 these differences must be very slight. Dr. Jermain Creighton^ 

 has shown that egg-albumin, a native protein and a very direct 

 product of living matter, can distinguish in its selective affinity 

 between iron in the trivalent and iron in the divalent state. 

 Dr. Creighton has found that egg-albumin apparently forms 

 a union with the ferri-ion whether that be as in ferric chloride 

 or in soluble Prussian Blue, (pottassium ferri-ferro-cyanide), 

 both of which have trivalent iron as a cation ; or in potassium 

 ferricyanide in which trivalent iron is part of a complex anion. 

 Some late work has sho^vn the iron in haemoglobin to be 

 the ferri-ion: so that it would appear that the jDoint is not 

 whether iron is cation or anion but whether it is tri- or di-valent.. 

 The difference is physico-chemically very slight, and yet the 

 albumin takes cognizance of it. In accordance with these views 

 some pharmacologists assert that simple anaemia is cured only 

 by ferric salts. Dr. Creighton has further shown that even 

 gelatine exhibits analogous selective affinities. Here, I think, 

 we are in presence of some very important facts as indicating 



1. Creighton, H, J. M. : Trans. N. S. Inst. Science xiii, (2), 61—75, (1911-1912). 



