NATURE OF THE NERVE IMPULSE. 



117 



that cell. However this may be, there is excellent experimental 

 evidence to show that the physiological integrity of the axis cylinder 

 depends upon its connection with its corresponding nerve cell. This 

 view dates from the interesting work of Waller,* who showed that 

 if a nerve be severed the peripheral stump, containing the axis cyl- 

 inders that are cut off from the cells, will degenerate in a few days. 

 The process of degeneration brought about in this way is known 

 as secondary or Wallerian degeneration. The central stump, on 

 the contrary, remains intact, except for a short region immediately 

 contiguous to the wound, for a relatively long period, extending 

 perhaps over years. Waller, therefore, spoke of the nerve cells as 

 forming the nutritive centers for the nerve fibers, and this belief 

 is generally accepted. In what way the cell regulates the nutrition 

 of the nerve fiber throughout its whole length is unknown. Some of 

 the cells in the lumbar spinal cord, for instance, give rise to fibers of 

 the sciatic nerve which may extend as far as the foot, and yet 



Fig. 50. Diagram to show the direction of degeneration on section of the anterior 

 and the posterior root, respectively. The degenerated portion is represented in black. 



throughout its whole length the nutritive processes in these fibers 

 are dependent on influences, of an unknown kind, emanating from 

 the nerve cells to which they are joined. By means of his method 

 Waller investigated the location of the nutritive centers for the 

 motor and sensory fibers of the spinal nerves. If an anterior 

 root is cut the peripheral ends of the motor fibers degenerate 

 throughout the length of the nerve, while the fibers in the stump 

 attached to the cord remain intact; hence the nutritive centers 

 for the motor fibers must lie in the cord itself. Subsequent histo- 

 logical work has corroborated this conclusion and shown that the 

 motor fibers of the spinal nerves take their origin from nerve cells 

 lying in the anterior horn of gray matter in the cord, the so-called 

 motor or anterior root cells. If the posterior root is cut between 



* Waller, " Miiller's Archiv," 1852, p. 392; and "Comptes rendus de 

 1'Acad. de la Science," vol. xxxiv, 1852. 



