ELECTBICAL PHENOMENA IN NEKVES. 



779 



that the nutrition of motor nerves is controlled by the ganglia in the 

 spinal cord, while the ganglion on the posterior root controls the nutri- 

 tion of the sensory fibres. Such ganglia controlling nutrition are spoken 

 of as trophic centres, and nerves, therefore, which are separated from 

 their trophic centres undergo permanent degeneration. The nature of 

 the influence exerted by trophic centres is, however, entirely unknown 

 (Fig. 330). 



If a nerve be divided and the divided ends again brought into con- 

 tact Ity means of sutures, regeneration takes place, and after a varying 

 time the nerve is again capable of conducting impulses. 



A B C D 



FIG. 830. DIAGRAM OF THE ROOTS OF A SPINAL NERVE SHOWING THE 

 EFFECTS OF SECTION. (Landois.) 



The black parts represent the degenerated parts. A, section of the nerve-trunk beyond the ganglion ; . 



B, of the anterior root, and C, of the posterior root ; D, excision of the ganglion ; a, anterior, p, posterior 

 roots ; g, ganglion. 



III. THE ELECTRICAL PHENOMENA IN NERVES. 



As in muscles, evidence of the presence of a constant electrical 

 current rna3 r be found in nerves. If a section of a nerve be removed 

 from the body and placed upon non-polar izable electrodes in connection 

 with a sensitive galvanometer, a strong electrical current may be observed 

 when the transverse section of the nerve is placed in contact with one 

 of the electrodes and the surface in connection with the other. The 

 current will then pass from the longitudinal section to the transverse 

 section; or, in other words, the natural surface will be positive and the 

 artificial surface negative. The nearer one electrode is to the equator, 

 and the other to the centre of the transverse section, the stronger will be 

 the current produced, and when two points on the surface at equal 

 distance from the equator are connected with the galvanometer no 

 current is obtained. The electro-motor force of the strongest nerve- 

 current has been placed at 0.02 of the Daniells' element. 



Like muscle, again, the natural current of nerve undergoes a 

 negative variation when the nerve is artificially stimulated. If a section 

 of nerve be so connected with a galvanometer as to develop a strong 

 current, and it then be stimulated either by the application of electricity 

 or a chemical or mechanical stimulus, the nerve-current will be found to 



