THE INHIBITION OF REFLEX ACTIONS. 581 



desired to move, to others. But with practice the indepen- 

 dent movements become easy. So, too, the localizing power 

 of the skin can be greatly increased by exercise (p. 560) as 

 one sees in blind persons, who often can distinguish two 

 stimuli on parts of the skin which are so near together as to 

 give only one sensation to other people. Such phenomena 

 depend on the fact that the more often a nervous impulse has 

 traveled along a given road in the gray matter, the easier 

 does its path become, and the less does it tend to wander 

 from it into others. We may compare the gray matter to a 

 thicket; persons seeking to beat a road through from one 

 point to another would keep the same general direction, 

 determined by the larger obstacles in the way, but all would 

 diverge more or less from the straight path on account of 

 undergrowth, tree trunks, etc., and would meet with consid- 

 erable difficulty in their progress. After some hundreds had 

 passed, however, a tolerably beaten track would be marked 

 out, along which travel was easy and all after-comers would 

 take it If instead of one entry and one exit we imagine 

 thousands of each, and that the paths between certain have 

 been often traveled, others less, and some hardly at all, we 

 get a pretty good mental picture of what happens in the 

 passage of nervous impulses through the gray matter of the 

 cord; the clearing of the more trodden paths answering to 

 the effects of use and practice. The human cord and that 

 of the frog must not, however, be looked upon as pathless 

 thickets at the commencement; each individual inherits 

 certain paths of least resistance determined by the structure 

 of the cord, which is the transmitted material result of the 

 life experience of a long line of ancestry. 



The Inhibition of Reflexes. Since it is possible, as by 

 strychnine, to diminish the resistance in the gray matter, it 

 is conceivably also possible to increase it, and diminish or pre- 

 vent reflexes. Such is found to be actually the case. We can 

 to a great extent control reflexes by the will; for example, 

 the jerking of the muscles which tends to follow tickling: 

 and it is found that after a frog's brain is removed it is much 

 easier to get reflex actions out of the spinal cord. Certain 

 drugs, as bromide of potassium, also diminish reflex excita- 



