506 THE NERVOUS SYSTEM 



has also been found that the Nissl granules which are present in the cell body 

 of resting cells decrease in number and show evidence of disintegration in 

 cells that have been stimulated for several hours. 



The nerve processes or fibers are primarily conducting structures. But 

 their fibers are susceptible to artificial stimulation, as shown in the previous 

 chapter, that is, they are irritable. They are influenced by certain changes in 

 the environment, but they do not show evidence of fatigue upon prolonged 

 functional activity. 



Nutritive Influence of the Cell Body over its Processes Wallerian 

 Degeneration. The control of the cell body over the nutrition of the 

 cell processes is demonstrated by the changes which occur when these proc- 

 esses are severed from connection with the cell body. Under such conditions 

 the axis-cylinder process completely degenerates. Ho well and Huber have 

 followed the degenerative changes in medullated nerve fibers. The medullated 

 fiber in the course of three or four days, in mammals, breaks up into elliptical 

 segments of myelin, containing small fragments of the axis-cylinder. These 

 changes in the cut-off section of nerve occur simultaneously throughout its 

 whole extent. In the course of a few weeks regenerative changes begin, ap- 

 parently under trophic influence of the nuclei of the primitive sheath. These 

 nuclei increase in number and form small masses of protoplasm which ulti- 

 mately produce a strand of embryonic protoplasm, which is described as the 

 " band fiber." If the ends of the sectioned nerve have originally been brought 

 together and sutured in place, then the axis-cylinder processes of the portion of 

 the nerve fiber still attached to the cell body will grow down into the peripheral 

 fibers, thus forming new axis-cylinder processes along the course of the band 

 fiber. If the stumps of the nerves are not so brought together, then apparently 

 the band fiber again degenerates, especially in adult tissues, though it has been 

 claimed by Bethe and others that complete regeneration of the peripheral 

 fiber will take place in very young animals. Even if complete regeneration 

 takes place in the peripheral fiber, unless connection is established between it 

 and the central end of the fiber it will ultimately disintegrate and can only 

 temporarily carry on any physiological function. 



The central end of the divided nerve, that is, the part maintaining con- 

 nection with the cell body, usually degenerates for a few nodes only, then re- 

 generation and growth of the original stump proceed. Instances are observed 

 in certain cases where the degeneration of the entire central fiber, including its 

 cell body, takes place. This happens particularly in those relations where the 

 original neurone forms a link in a conducting path. 



In conclusion, one may infer that the cell body exercises a nutritive or 

 trophic control over the protoplasm of its branches, just as we have 

 already seen the neurone as a whole exercises trophic control over 

 the nutritive processes taking place in the tissue to which its branches are 

 distributed. 



