CHEMICAL SIGNS OF IRRITABILITY 51 



not only the volume ratio between the axis cylinder 

 and sheath, but also the exact coefficient of the tempera- 

 ture radiation in the sheath. In this connection it is 

 interesting to note the very recent work of Snyder, who 

 showed that a smooth muscle also, when measured 

 in the same way, failed to produce heat during its con- 

 traction. But no one doubts that during muscle work 

 the metabolic activity is greatly accelerated. He used, 

 by the way, exactly the same technique as Hill. Either, 

 then, heat is produced, but owing to some circumstance 

 it is not detected, or else it is not produced. Now, most 

 muscles certainly produce heat when they work, and 

 it is probable that smooth muscle does so also. It is 

 not to be supposed that the muscles are a perfectly 

 reversible engine. On the contrary, we all know that 

 we become warm when we exercise. When, then, it is 

 reported that smooth, muscle produces no heat when it 

 contracts, we are at once skeptical of the method which 

 gives such a result. Consequently, therefore, while the 

 method for the detection of the nerve heat on stimula- 

 tion appears to be a competent method, we do not feel 

 certain that this is the case. 



But suppose we grant that the results are correct 

 that nerves produce no heat when they are excited- 

 does that mean that there is no chemical change occur- 

 ring in the nerve ? Is this fact conclusive evidence that 

 these results of a positive kind which we have adduced, 

 showing that chemical changes do occur in the nerves, 

 are, after all, due to some secondary cause or to some 

 undiscovered errors of technique on our part? Cer- 

 tainly this is not the case, for it is quite possible for 

 chemical changes to occur without liberating more than 



