76 A CHEMICAL SIGN OF LIFE 
centro-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 Kino Cras, Limulus 
polyphemus, FEMALE 
Amount of CO, 
Portion of Nerve Temperature} Given Off by ro mg. 
Degrees C. of the Nerve in 
1o Minutes 
Whole............. 17.8 2.6X107—7g. 
Proximal.......... 22.5 3.0X107 7g, 
Distalias cae cornered 22 5.0X10— 7g. 
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 
