i8 COMPARATIVE PHYSIOLOGY 



processes. On the basis of a prolonged series of experiments 

 on the cardiac and striped muscle of the frog, the heart muscle 

 of Pecten and several species of elasmobranch fishes, Mines 

 has attempted the classification of kations under three headings, 

 combining, nomadic, and polarising ions. Mines postulated 

 (i) that the normal activity of the muscular apparatus depends 

 on the maintenance of a certain degree of permeability at some 

 cell-surface ; (2) that the permeability of the cell-membrane 

 depends partly on the chemical composition determined by the 

 combination {inter alia) of Ca (and Sr under experimental 

 conditions) with some constituent of the cell ; and partly on 

 the electrical potential between the two sides of the membrane 

 itself. This latter is supposed to be modified by {a) the ability 

 of certain ions (Na and K), the nomadic ions, to pass through 



Fig. 6. — Effect of removal of magnesium (b — a) on the perfused heart 

 of Pecten (Mines, Journ. Physiol. 43, 1912). 



it selectively ; and {h) the adsorption of certain other ions, 

 e.g, Mg, La . . . and Ce . . . to the surface itself, thereby 

 reducing or reversing its normal charge. The hydrogen ion 

 is regarded as acting sometimes in one way, sometimes in the 

 other. 



On the whole the indications of recent work are distinctly 

 favourable to the role which Mines postulates for calcium. 

 The effects of the other polyvalent ions are more obscure, since 

 Mg . . , Ce . . . , La . . . , etc., agree with Ca in depressing the 

 striped muscle of the vertebrate, and that of the crustacean 

 heart ; while their action is opposite to that of Ca in a large 

 number of instances in the case of cardiac and plain muscle. 

 Thus in Pecten, removal of Mg, which is apparently essential 

 to the heart-beat, produces systolic arrest, while removal of 



