CHEMICAL SIGNS OF IRRITABILITY 21 
Thus, since we know the original volume of the chamber in 
which the respiration took place and from which this minimum 
volume is withdrawn, and since we know the quantity of carbon 
dioxide contained in this volume, it is easily calculated how 
much carbon dioxide.is given off by the nerve during the known 
period. It should be understood that, in determining the mini- 
mum volume of gas taken from the respiratory chamber, a series 
of experiments was conducted in order to calculate both the mini- 
mum volume which just gives the precipitate and the maximum 
volume which does not give the precipitate for a known weight of 
the nerve for a known period of respiration. In Table I, in the 
Appendix (p. 128), columns 8 and 9 refer to these volumes 
calculated from experiments for 10 mg. of the nerve, for ten 
minutes. 
Medullated nerve fibers——We have repeated this 
experiment with the sciatic nerve of the frog, this nerve 
being a typical medullated nerve. The result showed, 
not only that medullated nerves also give off carbon 
dioxide, but that they give a quantity of about 5.5X 
10? g. for each 10 mg. of the nerve for the first ten 
minutes, which is a little less than was obtained from 
the non-medullated nerve. 
A large variety of nerves was tested to see whether or 
not all resting nerves give off carbon dioxide. As a 
result, we found no exception in any of them, although 
they vary quite widely in the rate at which they 
produce carbon dioxide. The following nerves were 
examined, and it will be noticed that the list includes 
all varieties, such as sensory, motor, vertebrate and 
invertebrate, medullated and non-medullated nerves. 
1. Motor Nerve: Oculomotor nerve of the skate (Raia 
ocallaia). 
2. Sensory NERVE: Olfactory nerve of the same. 
