INDEPENDENT IRRITABILITY OF MUSCLES. 251 



their combination, which by their expansion is able to 

 accomplish such gigantic work. 1 In muscle, too, carbon 

 and oxygen lie side by side in chemical unstable equi- 

 poise; and it is the irritation of the nerves which effects 

 the solution which destroys the equilibrium. An arrange- 

 ment such as that just described is called sen-sitir<\ 

 because even an insignificant disturbance is sufficient to 

 disturb the unstable equipoise and to develop force. The 

 muscle is therefore a sensitive machine. But the nerve 

 is in a yet higher degree sensitive, for the smallest dis- 

 turbance of its equipoise gives play to the forces within 

 it. But these forces are in themselves incapable of any 

 great effects. They would hardly be iudicable, were 

 not this sensitive machine, which we call the nerve, 

 connected with the machine, also sensitive, which we 

 call muscle, in such a way that the activity of the one 

 sets free the forces within the other. 



5. A sensitive machine is not equally sensitive to 

 ail possible disturbances. Dynamite 2 may be placed 

 on an anvil and hammered without exploding ; or, if 

 lighted with a cigar, it burns quietly out like a fire- 

 work. But when it comes in contact with the spark of 

 a percussion cap, it explodes, and develops its gigantic 

 forces. A nerve is sensitive to electric shocks, and to 

 certain mechanical, chemical, and thermic influences. 

 It is not sensitive to many other influences. The in- 

 fluences to which the nerve is sensitive we have called 

 irritants. A muscle is sensitive to electric shocks, to 

 certain mechanical, chemical, and thermic influences ; 



1 On these processes see Balfour Stewart ' On the Conservation 

 of Energy' (International Scientific Series, vol. vi.) ; and Cooke 

 on ' The New Chemistry ' (same series, vol. ix.). 



2 Dynamite is a mixture of nitro-glycerine wi'.h 'kieselguhr,' an 

 earth consisting of the shells of infusoria. 



