INFLUENCE OF THE CELL BODY OVER ITS PROCESSES 537 



or in bees after a day's work, show marked evidences of fatigue. These 

 evidences consist in the decrease in the size of the nucleus and the appearance 

 of vacuoles in its structure, also in the shrinking of the protoplasm of the cell, 

 which, in case of the cells of the spinal ganglia, draws away from its capsule. 

 If the cells are examined early in the morning the fatigue changes will not 

 be present, the cell having recuperated during the period of rest at night. It 

 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 r 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. Howell and Huber have 

 followed the degenerative changes in medullated nerve fibers. The medul- 

 lated 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 through- 

 out its whole extent. In the course of a few weeks regenerative changes be- 

 gin, apparently under trophic influence of the nuclei of the primitive sheath. 

 These nuclei increase in number and form small masses of protoplasm which 

 ultimately 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 to- 

 gether, then apparently the band fiber again degenerates, especially in adult 

 tissues, though it has been claimed by Bethe and others that complete regen- 

 eration of the peripheral fiber will take place in very young animals. Even if 

 complete regeneration should take place in the peripheral fiber, unless con- 

 nection were established between it and the central end of the fiber it would 

 ultimately disintegrate and could 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, includ- 



