606 THE HUMAN BODY. 



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 considerable 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 path? answering 

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

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

 thickets at the commencement ; each individual inherits cer- 

 tain paths of least resistance determined by the stricture of 

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

 experiences of a long line of ancestors. 



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 

 prevent 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 excit- 

 ability. If a frog's brain be removed and the animal's toe be 

 dipped into very dilute acid, it will be removed after a few 

 seconds; the time elapsing between the immersion and the 

 lifting of the foot is known as the reflex time; anything 

 diminishing reflex excitability increases this, as the stimulus 

 (which has a cumulative effect on the centre) has to act longer 

 before it arouses the cord to the discharging point. If the 

 sciatic nerve of the other leg be stimulated while the toe is in 

 the acid the reflex time is increased, or the reflex may fail 

 entirely to appear. This is one case of a general law, that 

 any powerful stimulation of one sensory nerve tends to in- 

 hibit orderly reflexes due to the excitation of another. A 



