50 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



since fatigue sets in more rapidly with the larger loads. (7) 

 Marked activity in one set of muscles the use of the leg muscles 

 in long walks, for example will diminish the amount of work 

 obtainable from other muscles, such as those of the arm. It is 

 very evident that the instrurhent may be used to advantage in the 

 investigation of many problems connected with gymnastics, diet- 

 etics, stimulants,* medicines, etc. 



A point of general physiological interest that has been brought out in con- 

 nection with the use of the ergograph calls for a few words of special mention. 

 Mosso found that if a muscle e. g., the flexor digitorum sublimis is stimu- 

 lated directly by the electrical current and its contractions are recorded by 

 the ergograph, it will give a curve similar to that figured above for the volun- 

 tary contractions, except that the contractions are not so extensive. Under 

 these conditions the muscle, when completely fatigued to electrical stimula- 

 tion, will respond to voluntary stimulation from the nerve centers. It 

 seems likely, as suggested by Hough, that this result is due mainly to the 

 fact that the electrical current cannot be applied to a muscle in its normal 

 position so as to excite uniformly all the constituent muscle fibers, although 

 it is also possible that what we call the normal or voluntary stimulus is more 

 effective or, to use a physiological term, more adequate to the muscle fibers 

 than the electrical shock. On the other hand, after fatigue from a series 

 of voluntary contractions it has been observed that the muscle will still 

 give contractions if stimulated directly by electricity. This fact has been 

 interpreted to mean that, in the neuromuscular complex involved in a mus- 

 cular contraction namely, motor nerve cell, motor nerve fiber, and muscle 

 fiber the ordinary fatigue curve obtained from the ergograph does not repre- 

 sent pure muscle fatigue, but fatigue of the neuromuscular apparatus as a 

 whole. Wedenski has called attention to the fact that in the neuromus- 

 cular apparatus the motor end-plate is a sensitive link in the chain, and 

 that, when the nerve is stimulated strongly with artificial stimuli at least, 

 this structure falls into a condition in which it fails to conduct the nerve 

 impulse to the muscle. It may be, therefore, that in sustained voluntary 

 contractions the end-plate or the specialized receptive substance in which 

 the nerve fibers terminate fails first, and is directly responsible for the failure 

 of the apparatus to perform further work.f That the fatigue in ordinary vol- 

 untary contractions affects the muscles before the motor nerve centers is 

 indicated by the experiments of Storey. J Making use of a weight ergograph 

 and experimenting upon the abductor indicis, he found that after fatiguing 

 this muscle to voluntary contractions with a certain weight, removal of the. 

 weight enabled the individual to make contractions as high and as rapid as 

 before the fatigue. On the other hand, if, after removing the weight, the 

 muscle was stimulated electrically, the contractions were lower and slower than 

 before the fatigue. So far as our knowledge goes, therefore, fatigue as it 

 appears in sustained voluntary contractions is due probably primarily to 

 a loss of irritability in the muscle and in the receptive apparatus between nerve 

 and muscle. The motor nerve fibers do not fatigue, and as regards the motor 

 nerve centers, it is not possible as yet to say what may be their relative sus- 

 ceptibility to fatigue. A significant fact, reported by Piper, is that the motor 

 nerve centers when fatigued discharge their impulses at a rate of perhaps one- 

 half the normal. 



Sense of Fatigue. It should be noted in passing that in con- 

 tinued voluntary contractions we are conscious of a sense of fatigue 



*Schumberg, "Archiv f. physiol.," 1899, suppl. volume, p. 289, and 

 Palmen, "Skandinavisches Archiv fur Physiologic," 1910, 24, 168, 197. 



t For further evidence, see Burridge, "Journal of Physiology," 1911, 41, 285. 

 t Story, "American Journal of Physiology," 1903, 8, 355. 



