GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 127 



nerve-cells by which the muscles are excited to action during voluntary mus- 

 cular work. 1 This fact, that the nerve-cells give out before the muscles, ex- 

 plains the apparent contradiction, that a muscle fatigued by electric excitations 

 can be voluntarily contracted, and when the power to voluntarily contract the 



FIG. 55. Effect of fatigue on voluntary muscular contractions. The flexor muscles of the second 

 finger of left hand were voluntarily contracted once every two seconds, and always with the utmost 

 force. The weight raised was four kilograms. 



muscles has been stopped by fatiguing voluntary work the muscles will respond 

 to electrical excitation. It is undoubtedly of advantage to the body that the 

 nerve-cells should fatigue before the muscles, for the muscles are thereby pro- 

 tected from the injurious effects of overwork, and are always ready to serve the 

 brain. 2 It may be added that nerve-cells not only fatigue more quickly, but 

 recover from fatigue more rapidly than the muscles. 



(e) Effect of Temperature upon Muscular Contraction. Heat, within certain 

 limits, increases the irritability and conductivity of muscle-tissue, and at the 

 same time has a favoring influence upon those forms of chemical change which 

 liberate energy. The effect of a rise of temperature, as shown by the myo- 

 gram, is a shortening of the latent period, an increase in the height of contrac- 

 tion, and a quickening of the contraction and relaxation, the whole curve being 

 shortened. Of course there is an upper limit to this favoring action, since, at a 

 certain temperature, about 45 C. for frog's muscle and about 50 C. for the 

 muscles of warm-blooded animals, heat-rigor begins, and this change is accom- 

 panied by a loss of all vital properties. Cold can be said, in general, to pro- 

 duce effects the opposite of those of heat ; as the muscle is cooled, the latent 

 period, the contraction, and the relaxation, are all prolonged. 



Nevertheless, the effect of temperature is not a simple one (see Fig. 56). If 



1 Lombard : Archives Italiennes de Biologie, xiii. p. 1 ; or American Journal of Psychology, 

 1890, p. 1 ; Journal of Physiology, 1892, p. 1 ; 1893, p. 97. 



2 Waller: Brain, 1891, p. 179. 



