1863.] On the intimate Structure of Muscular Mire. 75 



we can expect this. We have not the remotest conception of the 

 nature of the active principle which maintains gravity or any other 

 force in operation, though we may with advantage study the laws which 

 govern these forces. All beyond this must at present be theory and 

 speculation. And so with the voluntary muscles ; we have advanced a 

 step in knowledge if we have gained a notion of their mechanical con- 

 struction, but I have no more expectation of determining the nature of 

 the subtle agency which sets the system at work through the nerves 

 than I have of seeing the changes which occur in the molecules of an 

 iron wire during the transit of an electric current through it. 



