NATURE OF THE NERVE IMPULSE. 



117 



action current is similar in all nerves examined; the reactions to 

 artificial stimuli are essentially similar. Moreover, nerves of, 

 one kind may be sutured to nerves of another kind, and, after re- 

 generation has taken place, the reactions are found to be deter- 

 mined solely by the place of ending (see p. 76) . i 

 The Nutritive Relations of the Nerve Fiber and Nerve Cell. 

 In recent times in accordance with the so-called neuron doctrine 

 (see p. 123) every axis cylinder has been considered as a process of; 

 a nerve cell, and therefore as a part, morphologically speaking, of 

 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 their 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. These influences may 

 consist simply in the effect of constant activity; that is, in the 



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

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



