168 GENERAL PROPERTIES OF LIVING TISSUES 



VI 



IRRITABILITY AND CONDUCTIVITY 



IRRITABILITY is the power of discharging energy 

 on stimulation. The form in which the kinetic 

 energy of muscle appears is partly mechanical 

 work (the visible contraction) and partly molec- 

 ular, heat, chemical action, and electricity. 

 In the nerve, the kinetic energy is wholly molec- 

 ular; an electromotive force is generated, prob- 

 ably heat is set free (though this statement 

 which is based simply on analogy is frequently 

 disputed), and a molecular change the nerve 

 impulse arises at the seat of stimulation. In 

 both muscle and nerve, by virtue of their con- 

 ductivity, the change induced by stimulation is 

 as a rule not limited to the region stimulated, but 

 passes in both directions along each stimulated 

 fibre. In neither muscle nor nerve can the 

 changes in energy spread transversely ; they are 

 limited to the muscle- or nerve-fibre in which 

 they arise. 



It will be shown that conductivity and irrita- 

 bility are essentially different functions. 



