MEMBRANE CHANGES DURING STIMULATION 351 



effect. Hence, their removal, independently of other 

 conditions, is favorable to recovery. But the main 

 factor in the reversal of the effects of stimulation, normal 

 or abnormal, appears to be the synthetic or structure- 

 forming (formative) metabolism of the protoplasm. 

 When the metabolic rate is rapid, recovery from stimula- 

 tion or injury is prompt and complete, and vice versa. 

 The difference between the recuperative and regenerative 

 powers of young and old individuals in higher animals 

 illustrates this condition. 



STIMULATING EFFECTS OF PERMEABILITY-INCREASING 



AGENTS 



Chemical or physical agents whose primary effect is 

 to increase the permeability of the cell surface to water- 

 soluble substances have as a class a strongly stimulating 

 action on many irritable forms of protoplasm. The 

 physiological effects produced by salts and combinations 

 of salts in isotonic solution illustrate this very clearly. 

 Thus pure solutions of neutral alkali salts, e.g., NaCl, 

 have in general a rapid permeability-increasing action 

 on living cells; this effect, if unreversed, is equivalent 

 to toxic, and is prevented by the addition of a small 

 proportion of CaCla or similarly acting salt to the solu- 

 tion; the antagonistic or '' an ti- toxic" action of the 

 latter salt is to be referred chiefly to this preventive 

 influence, which is equivalent to protective or anti- 

 cytolytic. Correspondingly, pure solutions of Na salts 

 cause stimulation or activation in many cells; and this 

 effect also is prevented by addition of calcium in propor- 

 tions similar to those required to prevent increase of per- 

 meability. The twitching of vertebrate skeletal muscle 



