38 OF VITAL MOTION. 



also involved in the development of nervous influence ; 

 and in this respect, as in other varieties of force, the 

 nervous influence may be regarded as the exponent of 

 a certain condition of change in matter. 



For these reasons it must be admitted that ordi- 

 nary physical agencies constitute a part, at least, 

 of nervous power ; nay, more, that the degree 

 of the one is commensurate with that of the others. 

 It is impossible, moreover, to conceive the idea of 

 ordinary force being present and inoperative ; and it 

 can scarcely be imagined that the nervous power 

 which is superadded to the more commonplace agents 

 should have a different law of action, otherwise the 

 one might negative the other. Whatever difference 

 of essence, reason and experience argue a community 

 of operation ; for, so far as the capillary vessels are 

 concerned, the effects of an insufficient or excessive 

 supply of nervous influence are similar to those which 

 attend equivalent alterations in the intensity of 

 ordinary force. Such, indeed, are the facts already 

 cited in connexion with the history of fear and joy 

 as to allow it to be supposed that the nerves act upon 

 the capillaries not by the sur-addition of any new 

 agency, but by means of that which is already in 

 operation, by that, namely, which is the necessary 

 exponent of the molecular changes in the material 

 part of the nervous substance. 



The metaphysical arguments which concern the 



