THE DIRECTION OF MOTION. 247 



which offers the least resistance to a discharge ; and is there- 

 fore the one along which a feeble force produces motion. 

 A sentiment or passion that is somewhat stronger, affects 

 not only the heart but the muscles of the face, and espe- 

 cially those around the mouth. Here the like explanation 

 applies; since these muscles, being both comparatively 

 small, and, for purposes of speech, perpetually used, offer 

 less resistance than other voluntary muscles to the nervo- 

 motor force. By a further increase of emotion the respira- 

 tory and vocal muscles become perceptibly excited. Final- 

 ly, under strong passion, the muscles in general of the 

 trunk and limbs are violently contracted. Without saying 

 that the facts can be thus interpreted in all their details (a 

 task requiring data impossible to obtain) it may be safely 

 said that the order of excitation is from muscles that are 

 small and frequently acted on, to those which are larger and 

 less frequently acted on. The single instance of laughter, 

 which is an undirected discharge of feeling that affects first 

 the muscles round the mouth, then those of the vocal and 

 respiratory apparatus, then those of the limbs, and then 

 those of the spine;* suffices to show that when no special 

 route is opened for it, a force evolved in the nervous centres 

 produces motion along channels which offer the least resist- 

 ance, and if it is too great to escape by these, produces mo- 

 tion along channels offering successively greater resistance. 

 Probably it will be thought impossible to extend this 

 reasoning so as to include volitions. Yet we are not without 

 evidence that the transition from special desires to special 

 muscular acts, conforms to the same principle. It may be 

 shown that the mental antecedents of a voluntary move- 

 ment, are antecedents which temporarily make the line 

 along which this movement takes place, the line of least re- 

 sistance. For a volition, suggested as it necessarily is by 

 some previous thought connected with it by associations 



* For details see a paper on " The Physiology of Laughter," published in 

 MacmillarCs Magazine for March 1860. 



