18 A CHEMICAL SIGN OF LIFE 
so scanty, was partly responsible for this conclusion.” 
In order to test the correctness of such an assumption, 
we have studied quantitatively the output of carbon 
dioxide from various lengths of nerves which are known 
to be free from nerve cells and which have almost no 
connective tissue in them. For this purpose the claw 
nerve from the spider crab was selected. 
Nerve fibers are of two kinds, called respectively 
medullated and non-medullated nerves. The essential 
conducting parts of these are alike, but the medullated 
fibers have lying about the conducting core of the fiber a 
white, glistening, fatty matter called the medulla, or 
myelin sheath. Most of the nerves going to voluntary 
muscle in the higher vertebrates are medullated; but 
the nerves to the viscera are often non-medullated 
and the nerves of the invertebrates are usually non- 
medullated. This medullary sheath is evidently some- 
thing which is found in those nerves which it is important 
should conduct very quickly and which should not be 
fatigued by conduction, and it is clear that the medulla- 
tion is an improvement which has not yet been universally 
introduced. The function of this sheath is probably 
nutritive. But in any case it is important, if we wish to 
avoid any complication which it may introduce into the 
physiology of the nerve, to examine both medullated and 
non-medullated nerves. And that we have done. 
Non-medullated nerve fibers ——When an isolated claw 
nerve of the spider crab is placed in the right chamber 
t Indeed, Bayliss attributes our results, which are soon to be detailed, 
to the presence of the connective tissue cells around the fiber, so firmly 
convinced does he appear to be that only nucleated parts of cells respire. 
For further consideration of this objection see p. 33. 
