576 THE NERVOUS SYSTEM 



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 physio- 

 logical function. Complete regeneration requires from three months to 

 two years. 



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- 

 ing its cell body, takes place. This happens particularly in those relations 

 where the original neurone forms a link in a conducting path. Such a con- 

 ducting neurone would no longer perform its proper function so would 

 atrophy just as would a muscle fiber when cut off from its nervous relations. 



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, for 

 example, the voluntary muscles. 



Specific Energy of the Nerve Impulses. We have already discussed 

 the fact that a nerve fiber, also its cell body, is irritable to various forms of 

 mechanical, electrical, and other stimuli. In the complex of activity of the 

 nervous system it is found that whatever the form of the external stimulus 

 applied to a nerve, the resulting nerve impulse produces the same effects 

 in the central nervous system. The reaction in consciousness is constant 

 and unvarying and independent of the character of the external stimulus. 



