THE DIRECTION OF MOTION. 245 



in, that part of the organism which is its seat; while a 

 mechanical movement implies an expenditure or loss of 

 force in that part of the organism which is its seat. Hence 

 if, as we find to be the fact, motion is habitually propagated 

 from those parts of an organism to which the external world 

 adds forces in the shape of nervous impressions, to those 

 parts of an organism which react on the external world 

 through muscular contractions, it is simply a fulfilment of 

 the law above enunciated. From this general con- 



clusion we may pass to a more special one. When there is 

 anything in the circumstances of an animal's life, involv- 

 ing that a sensation in one particular place is habitually fol- 

 lowed by a contraction in another particular place — when 

 there is thus a frequently-repeated motion through the or- 

 ganism between these places ; what must be the result as re- 

 spects the line along which the motions take place ? Restora- 

 tion of equilibrium between the points at which the forces 

 have been increased and decreased, must take place through 

 some channel. If this channel is affected by the discharge — 

 if the obstructive action of the tissues traversed, involves any 

 reaction upon them, deducting from their obstructive 

 power; then a subsequent motion between these two points 

 will meet with less resistance along this channel than the 

 previous motion met with; and will consequently take this 

 channel still more decidedly. If so, every repetition will 

 still further diminish the resistance offered by this route; 

 and hence will gradually be formed between the two a per- 

 manent line of communication, differing greatly from the 

 surrounding tissue in respect of the ease with which force 

 traverses it. We see, therefore, that if between a particular 

 impression and a particular motion associated with it, there 

 is established a connexion producing what is called reflex 

 action, the law that motion follows the line of least resist- 

 ance, and that, if the conditions remain constant, resistance 

 in any direction is diminished by motion occurring in that 

 direction, supplies an explanation. Without fur- 



