OP VITAL MOTION. 83 



As an objection to the notion that the voluntary 

 muscles may be made to contract by the direct opera- 

 tion of external cold, we must mention, however, a 

 statement often made by physiologists, that heat is 

 given out during the state of contraction. The fact 

 upon which this opinion is founded is the small increase 

 of temperature which takes place during violent exer- 

 cise, as of the forearm in the act of sawing, and this 

 fact is undeniable. The conclusion, however, is alto- 

 gether gratuitous. In an act of this kind, indeed, it 

 is not with mere muscular contraction that we have 

 to do, but with an alternate series of contractions and 

 dilatations ; and it is quite as probable that the eleva- 

 tion of temperature is connected with the latter phe- 

 nomena as with the former. Nay, it is more so : 

 for when the limb is wearied and the action at an 

 end, and when also the temperature is highest, the 

 condition of the parts is much more nearly allied to 

 relaxation than to any other state, while the fact that 

 the increased heat continues for some time after the 

 cessation of the contractions, shows that it is not to 

 these that we must look for its immediate cause. 



It is difficult to procure any direct evidence 

 of the action of light upon the voluntary muscles, for 

 this is so mixed up with other actions : but, at the 

 same time, we may notice two circumstances which 

 seem to bear upon the question. The first of these is 



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