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PHYSIOLOGY: 5. TASHIRO 
Granting that the absence of fatigability in the nerve, as measured 
by ordinary methods, may not be a question of absence of metabolism, 
but merely of the speed at which breaking and repairing processes of the 
tissue come to equihbrium, one may still ask how we explain the lack of 
heat production. Snyder reported very recently ^ that a smooth muscle 
failed to show any sign of heat formation during contraction, which, no 
one doubts, increases carbon dioxide production. When we dip a zinc 
rod into copper sulphate solution we observe heat formation, but that 
with proper arrangements, as in a Daniell cell, the reaction goes on 
practically isothermally, all the chemical energy being converted into 
electrical energy. I do not think that we should ignore the increase of 
carbon dioxide production in the nerve on stimulation because of the 
fact that we cannot detect heat formation. 
There are some physiologists who admit that the living nerve should 
be chemically active to maintain a 'normal' condition, Uke any other 
living tissue, but who hold that the increase in carbon dioxide pro- 
duction on stimulation must be a secondary effect due to primary physical 
changes. We will consider this question in a quite different way. 
The state of a nerve fiber depends upon three conditions: the degree 
of irritability, the direction of the impulse and the rate of the impulse. 
The relation of these conditions to chemical activity in the nerve is 
analyzed in the following way: 
1. Degree of irritability and carbon dioxide production. 
We have already cited the fact that chemical reagents which modify 
the degree of excitability invariably modify the rate of carbon dioxide 
production in the same proportion. The chemical activity in the 
nerve fiber seems to determine the state of nerve excitability. 
2. The direction of nerve impulse and carbon dioxide production. 
If one takes nerve bundles containing only sensory fibers, which 
conduct the normal nerve impulse in a central direction, the portion 
of the nerve nearer the natural source of the nerve impulse (i.e., nearer 
the end organ) gives more carbon dioxide than the portion away from it. 
There is a gradient of carbon dioxide production in the unstimulated 
nerve. This gradient of chemical condition seems to determine the 
direction of nerve impulse. Many experiments made on various kinds 
of pure nerve fibers enable us to generalize this by saying that the normal 
nerve impulse passes toward a point of lower carbon dioxide production. 
3. Rate of the nerve impulse and carbon dioxide production. 
There seems to exist a close relation between the rate of nerve im- 
pulse and carbon dioxide production in the resting nerve, if one com- 
