5*78 A MANUAL OF PHYSIOLOGY 



the frog about 8 to 10 per second, and asserted that by means 

 of the capillary electrometer (p. 538) 8 electrical oscillations 

 per second could be demonstrated in voluntarily contracted 

 muscle. But they seem to differ in amplitude or abrupt- 

 ness from the electrical changes produced in experimental 

 tetanus. . For secondary tetanus (p. 644) is not caused by 

 muscle in voluntary contraction, except (and even this is 

 doubtful) just at the beginning. While, therefore, it cannot 

 be denied that the voluntary contraction is discontinuous, 

 in the sense that it is not a perfectly smooth and uniform 

 tonic contraction, we still lack a decisive proof that it \i 

 maintained by a strictly intermittent outflow of nervous 

 energy, and not by a continuous outflow causing a sustained' 

 contraction, which, it may be, remits and is reinforced at 

 intervals. In this connection it is interesting that chemical 

 stimulation, for example by ammonia, which certainly sets 

 up a condition of sustained contraction, does not cause 

 secondary tetanus. In any case, some voluntary contractions, 

 namely, the shortest possible, do not seem to be tetanic. For 

 a voluntary movement can be executed in T V to Y V f a second, 

 which, if we take the greatest frequency of discharge in 

 natural tetanus that has been suggested, would allow time 

 only for a single oscillation, caused by a single impulse. 



(3) Thermal Phenomena of the Muscular Contraction. When 

 a muscle contracts its temperature rises ; the production of 

 heat in it is increased. This is most distinct when the 

 muscle is tetanized, but has also been proved for single con- 

 tractions. The change of temperature can be detected by 

 a delicate mercury or air thermometer ; and, indeed, a 

 thermometer thrust among the thigh-muscles of a dog may 

 rise as much as i to 2 C. when the muscles are thrown 

 into tetanus. In the isolated muscles of cold-blooded animals 

 the increase of temperature is much less ; and electrical 

 methods, which are the most delicate at present known, 

 have generally been used for its detection and measurement. 



They depend either upon the fundamental fact of thermo-elec- 

 tricity, that in a circuit composed of two metals a current is set up if 

 the junctions of the metals are at different temperatures ; or upon 

 the fact that the electrical resistance of a metallic conductor varies 

 with its temperature. 



