848 CENTRAL NERVOUS SYSTEM 



of regeneration, by means of which the nerve cells reestablish a func- 

 tional connection with the muscles. If such reestablishment is impos- 

 sible, as in the case of amputation, disuse may be accompanied by an 

 atrophy of the neurons so that the number of cells in the ventral horn 

 of the cord decrease and their fibers degenerate. 



While degeneration of the nerve fiber is doubtless influenced greatly 

 by the presence of the nerve cell body, a major part in the process is 

 attributed to the cells of the neurilemmal sheath. The process of re- 

 generation occurs coincidently with that of degeneration. It commences 

 with activity of the nuclei of the neurilemma, which divide rapidly and 

 form about themselves strands of protoplasm which replace the frag- 

 ments of the degenerated fiber as it is absorbed. While it has been 

 claimed that the fibers of the new nerve are actually formed by the neu- 

 rilemma, it seems more probable that they grow out along the strands 

 of this tissue from the central stump of the nerve, the neurilemma fur- 

 nishing them with guidance, support, and perhaps nutrition as they 

 grow. The importance of the neurilemma in the process is supported 

 also by the fact that in the spinal cord and brain, where the neurilemma 

 is absent, nerve fibers do not display an ability to regenerate. And herein 

 lies an important principle in prognosis, for injuries to peripheral nerves 

 may be repaired by regeneration, whereas the severance of fibers in the 

 central nervous system results in a permanent injury which can be 

 corrected only by developing the use of new paths for the control of 

 the functions which are disturbed. 



It should be obvious that these facts form the basis for understanding 

 the clinical treatment of peripheral nerve injuries. Although the periph- 

 eral end of a severed nerve is doomed, its neurilemma will form the 

 guiding path in the establishment of a connection between the new fiber 

 and the muscle. It is consequently desirable to reunite the ends of the 

 fibers in order to facilitate the regeneration by allowing the nerve fibers 

 to establish a connection with the neurilemma of the peripheral stump. 

 Even when the stumps are not brought into direct contact it is very 

 remarkable to observe with what surety the new fibers grow out into 

 the intervening cicatrix and establish a relation with the neurilemmse 

 of the peripheral trunk. 



The phenomenon of degeneration has provided an invaluable tool for 

 the study of the anatomical relationships within the nervous system 

 and the function of its parts. By destroying the cells of a nerve center 

 and observing which fibers degenerate, one can ascertain the distribu- 

 tion of the fibers which arise from the center, while a study of the 

 disturbances in the activity of the animal produced by the lesion tells 

 us what functions are dependent on the neurons which are involved. 

 Since degeneration does not extend beyond the synapses of the neuron 



