CHEMICAL SIGNS OF IRRITABILITY 47 



these products are of such a nature that they have very 

 little physiological action. It is quite possible that 

 they are taken care of in the nerve, because it is vitally 

 necessary to animals that those of their nerves which 

 go to skeletal muscle, at any rate, shall not be easily 

 fatigued. There are also other tissues in which it is 

 perfectly certain that there is a rapid metabolism and 

 which also show a no less remarkable freedom from 

 fatigue. We may cite, for example, the contracting 

 wings of insects which vibrate at a rate as high as three 

 hundred vibrations per second, and yet these insects can 

 fly for hours continuously without muscular fatigue. 

 There is not the least doubt that these muscles which 

 are undergoing this tremendous activity without fatigue 

 are at the same time undergoing a very rapid metabolism. 

 All that is necessary to avoid fatigue is that the tissue 

 shall return each time after activity to its normal state. 

 The ordinary induction coil which we use in these experi- 

 ments only stimulates a nerve about one hundred times 

 a second, or about one-third as often as the insect's wing 

 muscles contract, so that more time is given for recovery 

 in the nerve than in these muscles. The lack of appar- 

 ent fatigue in nerves is not, then, any proof of the 

 absence of metabolism. 



When we examine nerves more closely and by more 

 delicate methods, we find unmistakable evidences of 

 fatigue in them. The only remarkable thing about them 

 is that they recover from that fatigue very rapidly. 

 Thus Gotch and Burch discovered in 1889 tnat # two 

 stimuli are successively applied to a nerve within 

 1/5,000 of a second, only a single nerve impulse is pro- 

 duced. One cannot generate a second impulse until 



