1 8 A CHEMICAL SIGN OF LIFE 



so scanty, was partly responsible for this conclusion. 1 

 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 

 medulla ted and non-medulla ted 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 



1 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. 



