ELECTROLYTES IN LIVING MATTER 105 



supports the stand with the nerve to bring about a twitching of the 

 muscle. If we may be guided by physical analogies, which 

 however, are not absolutely reliable, - - this mechanical stimulation 

 might be compared with the effect which a mechanical agitation has, 

 under certain conditions, upon an oil drop on the surface of a Na 2 CO 3 

 solution. In this case it may lead to a dissolution of a solid soap 

 film on the surface, or to an alteration of the surface, by bringing new 

 particles of both liquid media in contact. Thus phenomena of spread- 

 ing may be provoked by a slight mechanical agitation. Mechanical 

 stimulation is much more effective in nerves than in muscles. 



As a rule, heat is also mentioned as a stimulant though nobody 

 uses this form of energy for this purpose. The term " stimulation by 

 heat " is a misleading phrase, as we shall see presently. 



It appears from the foregoing that by the word " stimulation " we 

 mean a process which is unknown to us, which, however, seems to con- 

 sist after the data given in this lecture in the substitution of Na- or K- 

 ions for Ca, or "vice versa, in some colloidal (proteid or lipoid) compound 

 of the muscle or nerve, whereby some physical qualities of the colloidal 

 substances are changed. 



