CHAPTER XCI 

 THE NUTRITION OF NERVOUS TISSUE 



The Function of the Nerve Cell Body 



In the preceding chapter we considered the physiology of the nerve 

 fiber and the synapses in which it terminates. We must now inquire 

 what part the nerve cell body, containing the nucleus of the neuron, takes in 

 the conduction of the nervous impulse. In the crayfish it is possible to 

 remove the cell bodies from the motor neurons of the antenna without 

 disturbing the reflex connections of the nerve fibers. This can be done 

 because the motor neuron possesses a single axon which divides at some 

 distance from the cell body into two collaterals, one connecting with the 

 afferent neurons of the reflex arcs which control the movements of the 

 antennas, the other passing directly to the muscles of that organ. Cutting 

 away the part of the cephalic ganglion which contains the cell bodies 

 of these neurons does not interfere with the reflex excitation of the 

 muscles of the antenna?, provided that the continuity of the collaterals 

 is not destroyed. The nerve fibers, deprived of their cell bodies are able 

 to function normally for two or three days. Conduction does not then 

 depend primarily upon the presence of the nerve cell body. On the days 

 following the operation the reflex is elicited with greater and greater 

 difficulty, and fails altogether on the third or fourth day. The nerve cell 

 body is consequently necessary for maintaining the conductivity of its 

 nerve fibers, that is, it is concerned with the nutrition of the outlying parts 

 of the neuron. 



Degeneration and Regeneration of Nerve Fibers. 4 The nutritive func- 

 tion of the nerve cell is also illustrated by the phenomena w r hich follow 

 the section of a peripheral nerve trunk or of the tracts of fibers in the 

 central nervous system. "When a man's motor nerve is severed, the 

 excitability of the peripheral part may be increased for one or two days, 

 but will then decline rapidly and disappear completely by the end of 

 the second week. Microscopic examination reveals the fact that such 

 a nerve, the fibers of which are cut off from their cell bodies, has under- 

 gone degeneration. The fibers have broken up into ellipsoid segments 

 of myelin, each containing a piece of the axis cylinder, and these seg- 

 ments later fragment very irregularly into smaller pieces which are 

 eventually absorbed. In animals such as the rabbit, in which the process 

 occurs slowly, it can be seen that the degenerative changes begin at the 



846 



