76 A CHEMICAL SIGN OF LIFE 



centre-peripheral respiratory gradient in the claw nerve 

 of the spider crab, but also indicate very clearly that 

 this gradient exists in a normal nerve independent of 

 injury. 



Afferent fibers. The optic nerve of Limulus was 

 tried next. It is a non-medullated, long, apparently 

 uniform, nerve. It can be isolated in a length of four 

 or five inches without cutting it at either end, though 

 the task is rather laborious. It is important that the 

 peripheral end should be left intact with the eyes. This 

 is accomplished by cutting the shell about two inches 

 square around the eye. By gently lifting the eye with 

 the nerve, we can easily trace the nerve centrally up to 

 the brain without any injury. 



TABLE XI 



CARBON DIOXIDE PRODUCTION FROM DIFFERENT PORTIONS 

 OF OPTIC NERVE OF KING CRAB, Limulus 

 polyphemus, FEMALE 



When such a long stretch of the nerve is cut at both 

 ends simultaneously and is then divided at the middle 

 so as to furnish two parts of approximately the same 

 weight, and the rate of carbon dioxide production of 

 these two parts is compared, we find that the centro- 

 peripheral gradient discovered in the case of the claw 

 nerve is exactly reversed. In the optic nerve of Limulus 

 the proximal portion (nearer to the brain ring) gives 



