PHYSIOLOGY OF NERVE TISSUE. 79 



the parent cell, the fiber appears to be incapable of itself of main- 

 taining its nutrition. 



The relation of the nerve-cells to the nerve-fibers, in reference to 

 their nutrition, is demonstrated by the results which follow section 

 of the ventral and dorsal roots of the spinal nerves. If the anterior 

 root alone be divided, the degenerative process is confined to the 

 peripheral portion, the central portion remaining normal. If the 

 posterior root be divided on the peripheral side of the ganglion, de- 

 generation takes place only in the peripheral portion of the nerve. 

 If the root be divided between the ganglion and the cord, degenera- 

 tion takes place only in the central portion of the root. From these 

 facts it is evident that the trophic centers for the ventral and dorsal 

 roots lie in the spinal cord and spinal nerve ganglia, respectively, or, 

 in other words, in the cells of which they are an integral part. The 

 structural changes which nerves undergo after separation from their 

 centers are degenerative in character, and the process is usually 

 spoken of, after its discoverer, as the Wallerian degeneration. 



When the degeneration of the efferent nerves is completed, the 

 structures to which they are distributed, especially the muscles, un- 

 dergo an atrophic or fatty degeneration, with a change or loss of their 

 irritability. This is, apparently, not to be attributed merely to in- 

 activity, but rather to a loss of nerve influences, inasmuch as inac- 

 tivity merely leads to atrophy and not to degeneration. 



Reactions of Degeneration. In consequence of the degeneration 

 and changes in irritability which occur in nerves when separated 

 from their centers and in muscles when separated from their 

 related nerves, either experimentally or as the result of disease, the 

 response of these structures to the induced and the make-and-break 

 of the constant currents differs from that observed in the physiologic 

 condition. The facts observed under the application of these two 

 forms of electricity are of the greatest importance in. the diagnosis 

 and therapeutics of the precedent lesions. The principal difference 

 of behavior is observed in the muscles, which exhibit a diminished 

 or abolished excitability to the induced current, while at the same 

 time manifesting an increased excitability to the constant current ; 

 so much so is this the case that a closing contraction is just as likely 

 to occur at the positive as at the negative pole. This peculiarity 

 of the muscle response is termed the reaction of degeneration. The 

 synchronous diminished excitability of the nerves is the same for 



