EXCITATION AND CONDUCTION 77 



much less carbon dioxide than the distal portion (nearer 

 to the retina), where the impulses normally originate. 



Table XI shows the quantitative results. 



Sensory dendrites. These results were at first 

 surprising; but they became exceedingly interesting 

 when we took into consideration the functional or 

 developmental difference between the two nerves. 

 The claw nerve of the spider crab is believed to be 

 composed mainly of efferent fibers, while the optic 

 nerve of Limulus is an almost purely afferent nerve. 

 The direction of the normal nerve impulse in one 

 of these nerves is, therefore, exactly opposite to its 

 direction in the other. .Developmen tally speaking, 

 however, the distal portion of the optic nerve corre- 

 sponds to the proximal portion of the claw, nerve in that 

 these portions^ are in each case nearer the nerve cells 

 from which the fibers come. Thus our results with the 

 two opposing gradients may be subject to two alternative 

 interpretations. Either the metabolic gradient may 

 correspond to the developmental gradient, i.e., all the 

 portion nearer to the mother-cells may have a higher rate 

 of metabolism, or it may correspond to the functional 

 gradient, i.e., the nearer the portion is to the stimulus 

 the higher is the carbon dioxide production. 



This question will be automatically solved if we 

 study the metabolic gradient of a fiber whose functional 

 direction is opposite to its developmental direction; 

 e.g., an afferent nerve fiber lying peripherally to its 

 nerve cells i:e. r a--sensory dendrite should be studied. 

 Professor C. Judson Herrick kindly suggested that we 

 use a lateral line nerve, or an accessory lateral line 

 nerve, of a fish. In the carp and the catfish both of 



