30 ESSENTIALS OF PHYSIOLOGY. 







muscle attains a certain level, the proteins are coagulated and the 

 physical characters of the muscle are altered. It is for this reason 

 hat the muscles in fatigued animals pass into rigor mortis more rapidly 

 after death than the muscles of resting animals, since at the time of 

 death they already contain some lactic acid. That the accumulation 

 of lactic acid in muscle can cause fatigue is shown by the fact that the 

 contraction characteristic of fatigued muscle can be induced in the fresh 

 muscle by perfusing it with blood containing lactic acid. Fatigue is 

 thus brought about in part by the same cause as that which ultimately 

 leads to rigor mortis ; but the larger amount of acid produced in the 

 latter brings about an irreversible change in the proteins, causing the 

 death of the muscle, whereas the acid is gradually removed from a 

 fatigued muscle, which is thus restored to its normal condition. 



When a muscle is repeatedly stimulated through its nerve, it 

 gradually becomes fatigued and finally fails to contract. When this 

 point is reached the muscle will still contract, if directly stimulated, 

 so that its failure to contract when stimulated through its nerve must 

 be due to fatigue of either the nerve trunk or the nerve endings. It 

 can readily be shown that it is the nerve endings which become 

 fatigued in such an experiment. 



Two muscle nerve preparations are made, and the nerves are 

 continuously stimulated with a tetanising current, the passage of the 

 stimulus to one muscle being prevented by cooling a small portion of 

 the nerve near the muscle to C. ; in this way both nerves are 

 stimulated, but only one muscle is thrown into contraction. After a 

 short time the muscle which is contracting becomes fatigued and fails 

 to contract. The cooled nerve is then warmed, the stimulation being 

 continued, and the muscle of this preparation at once contracts 

 vigorously. Since both nerve trunks have been equally stimulated, it 

 is evident that the fatigue does not lie in the nerve trunks and must 

 therefore have its seat in the nerve endings. 



Experiments of this kind have shown that both medullated and 

 non-medullated nerves are practically incapable of fatigue, and even 

 after six hours of continuous excitation the nerves are just as excitable 

 as at the outset. 



VOLUNTARY CONTEACTION. 



The contraction of a muscle under the influence of the will is much 

 longer than a single twitch and represents a short tetanus, since even 

 the quickest movement which an individual can carry out voluntarily 

 lasts at least one-tenth of a second, and usually longer. The nature of 

 voluntary movement has recently been demonstrated by recording the 



