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 
fatigueinthem. The only remarkable thing about them 
is that they recover from that fatigue very rapidly. 
Thus Gotch and Burch discovered in 1889 that if 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 
