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 
itis 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 
