t54 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



than any artificial stimulus. But the alternative explanation that 

 the electrical stimuli cannot be applied to a muscle in situ, so as to 

 cause uniform excitation, and therefore uniform fatigue, of all the 

 fibres of the muscle, is more probable (Hough). 



It has been shown that the injection of the blood of an animal 

 exhausted by running or other muscular effort into the circulation 

 of a normal animal produces in the latter all the symptoms of fatigue. 

 Here the fatigue-producing substances will have the opportunity 

 of acting on both the central and the peripheral mechanisms. There 

 are reasons for believing that the fatigue process is fundamentally 

 the same in different tissues. The fatigue substances produced in 



Fig. 253. Influence of Mental Fatigue on Muscular Contraction, i, series of con- 

 tractions of flexors of middle finger before, and 2, series of contractions imme- 

 diately after, a period of three and a half hours' hard mental work. In both 

 cases the muscles were stimulated directly every two seconds by an electrical 

 current, and caused to raise a certain weight till temporary exhaustion occurred. 

 In the first series fifty-three contractions were found possible, in the second only 

 twelve (Maggiora). 



muscle, and not immediately eliminated or transformed during 

 active muscular exertion, may therefore very well be a factor in 

 inducing fatigue of the central nervous mechanisms in addition to 

 the formation of fatigue products, and the using up of necessary 

 material in these mechanisms themselves. Conversely, active 

 and long-continued mental exertion may occasion muscular fatigue 

 (Fig. 263). The sensation of fatigue is alluded to in Chapter XVIII. 



(d) The Influence of Drugs on the Contraction of Muscle. The total 

 work which a muscle can perform, its excitability and the absolute 

 force of the contraction, may all be altered either in the plus or the 

 minus sense by drugs. But in connection with our present subject 

 those drugs which conspicuously alter the form and time-relations of 

 the muscle-curve have most interest. Of these veratrine is especially 



