CHAP, ii.] THE CONTRACTILE TISSUES. 151 



reason, possibly the chief reason, why a muscle is increased by use, 

 that is to say, the loss of substance and energy caused by the 

 contraction is subsequently more than made up for by increased 

 metabolism during the following period of rest. 



Whether there be a third factor, whether muscles for instance 

 are governed by so-called trophic nerves which affect their nutri- 

 tion directly in some other way than by influencing either their 

 blood supply or their activity, must at present be left undecided. 



A muscle, even within the body, after prolonged action is 

 fatigued, i.e. a stronger stimulus is required to produce the same 

 contraction; in other words, its irritability may be lessened by 

 functional activity. Whether functional activity therefore is in- 

 jurious or beneficial depends on its amount in relation to the 

 condition of the muscle. It may be here remarked that as a muscle 

 becomes more and more fatigued, stimuli of short duration, such as 

 induction-shocks, sooner lose their efficacy than do stimuli of longer 

 duration, such as the break and make of the constant current. 



It is worthy of notice that a motor nerve is far less susceptible 

 of being fatigued by artificial stimulation than is a muscle; in 

 fact it seems extremely difficult to tire a nerve by mere stimula- 

 tion. In an animal poisoned by urari the sciatic nerve may be 

 stimulated continuously with powerful currents for even several 

 hours and yet remain irritable. So long as the urari is produc- 

 ing its usual effect, the muscles sheltered by it are not thrown 

 into contraction by the stimulation of the nerve and so are not 

 fatigued ; as the effect of the urari passes off, contractions make 

 their appearance in response to the stimulation of the sciatic 

 nerve, shewing that this, in spite of its having been stimulated 

 for so long a time, has not been exhausted. And other experi- 

 ments point to a similar conclusion. It would seem that the 

 molecular processes constituting a nervous impulse unlike those 

 constituting a muscular contraction, are of such a nature or take 

 place in such a way, that after the development of one impulse 

 the substance of the nerve fibre is at once ready for the develop- 

 ment of a second impulse. 



The sense of fatigue of which, after prolonged or unusual exer- 

 tion, we are conscious in our own bodies, is probably of complex 

 origin, and its nature, like that of the normal muscular sense of 

 which we shall have to speak hereafter, is at present not thoroughly 

 understood. It seems to be in the first place the result of changes 

 in the muscles themselves, but is possibly also caused by changes in 

 the nervous apparatus concerned in muscular action, and especially 

 in those parts of the central nervous system which are concerned 

 in the production of voluntary impulses. In any case it cannot be 

 taken as an adequate measure of the actual fatigue of the muscles ; 

 for a man who says he is absolutely exhausted may under excite- 

 ment perform a very large amount of work with his already weary 

 muscles. The will in fact rarely if ever calls forth the greatest 

 contractions of which the muscles are capable. 



