464 NERVES. 



there is a loss of power, or paralysis of motion, in the muscle sup- 

 plied by the nerve. If, on the other hand, a galvanic current is 

 passed through the nerve, there will follow contraction of the 

 muscle. Similarly a paralysis of sensation will follow division of 

 a sensory nerve ; and when such a nerve is stimulated, sensation 

 will result. When a nerve is thus divided, one portion will remain 

 in communication with the nerve-center, and is called the central 

 or proximal end ; while the other, which remains in communica- 

 tion with the periphery, is the distal or peripheral end. Stimu- 

 lation of the central end of a motor nerve produces no effect ; 

 while the motion referred to above results from stimulation of its 

 peripheral end. In the case of a sensory nerve, it is the reverse. 



Wallerian Degeneration. When a nerve is divided the 

 first result is a loss of its function. Afterward the nerve under- 

 goes Wallerian degeneration, which results in changes in its 

 structure that can readily be seen. Inasmuch as each nerve-fiber 

 develops from a cell which later nourishes it, if the connection 

 between the two is severed, the nerve-fiber undergoes Wallerian 

 degeneration, and in the case of a nerve which is made up of 

 nerve-fibers the whole nerve undergoes this change. This degener- 

 ation consists, in the case of medullated nerves, in the death of the 

 axis-cylinder, the breaking up of the medullary sheath into drops 

 of myelin, which are later absorbed, and the multiplication of 

 the nuclei of the primitive sheath. In non-medullated nerves 

 the only result is the death of the axis-cylinder. Degeneration 

 begins very soon after the section Avithin a day or two 

 and throughout the entire severed portion of the nerve at the 

 same time. Thus the course of a nerve, or a collection of 

 nerves, may be traced throughout its entire extent. These changes 

 are believed to be due to the severance of the nerve from its 

 trophic center. If an anterior root of a spinal nerve is divided, 

 the distal end, being separated from the gray matter of the cord 

 which is its center of nutrition, undergoes degeneration, while the 

 end which remains connected with the cord retains its integrity. 

 If a posterior root is divided between the cord and the ganglion, 

 the degeneration takes place between the cord and the ganglion ; 

 while if divided below the ganglion, the degeneration takes place 

 in that portion separated from the ganglion, showing that the 

 ganglion is the nutritive center for the posterior root. 



Regeneration of Nerves. If the two ends of a divided 

 nerve are brought together and retained there, a regeneration may 

 take place, and the new structure has all the properties of the 

 original nerve. It will be remembered that in the degeneration 

 which follows division of a nerve there is an increase in the 

 nuclei of the sheath. These form a continuous thread of pro- 

 toplasm within the old sheath, and around this protoplasm a new 

 sheath develops, the whole forming an embryonic nerve-fiber, 



