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AN AMERICAN TEX1-BOOK OF PHYSIOLOGY. 



cells by which the muscles are excited to action during the 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 he voluntarily contracted, and when the power to voluntarily contract the 

 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 



c 



FlG. f>0. — Schema of effect of temperature on height and form of contraction curve : a, contraction at 

 19° C. ; b, c, d, e,f, contractions made at intervals, each one at a lower temperature; g, h, contractions 

 at higher temperatures than 19° C, h being made when the temperature was 30° C. ; i, k, I, show a different 

 series of contractions, made as the temperature was increased from 30° C. toward the point at which the 

 muscle-substance coagulates (after Gad and Heymans). 



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

 striated muscles of warm-blooded animals, 53°— 58° C. for the non-striated 

 muscles of the bladder of the cat, 3 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. 60). If 

 during the cooling process a striated muscle of a frog be irritated from time to 

 time with single induction shocks, the height of the contractions does not con- 

 tinually grow less as one would expect.* The maximal height is obtained at 

 30° C, the height above this point being somewhat less, the irritability les- 

 sening as the coagulation-point is approached; from 30° C. to 19° C. the 

 height continually decreases, but from 19° to 0° C. the height increases, while 



'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. 

 'Waller: Brain, 1891, p. 179. 



S C. C. Stewart- American Journal of Physiology, 1900, iii. p. 25. 

 '•lad und Heymans: Arehivfur Anatomic wnd Physiologic, 1890, S. 73. 



