ON THE MAMMALIAN NERVOUS SYSTEM. 309 



The root, therefore, in this respect behaves, as far as these two experiments enable 

 us to judge, like the nerve. 



(c.) Changes following Excitation. In consequence of either direct excitation of 

 the root or its indirect excitation through the cord, the amount of the difference is 

 decreased, an after-effect following the passing away of the true excitatory change 

 similar in character to that seen in the nerve. 



The rising and falling in the amount of the difference when the anesthesia of the 

 animal is not sufficiently profound is rather more marked in the case of the posterior 

 root than in that of the mixed nerve. 



C. ANTERIOR ROOT. 



With regard to the anterior roots, we have only performed one experiment. There 

 are considerable difficulties in the way of connecting the anterior root satisfactorily 

 with the electrodes, so as to exclude all possibility of obtaining results which are 

 derived from electrical changes in the cord. These difficulties are mainly the shortness 

 of the root arid the characters of its anatomical relations with the cord, it being 

 difficult to obtain a sufficient length. Moreover, our purpose being rather to obtain 

 the evidence of excitatory than of resting electrical changes, the shortness of the root, 

 when combined with the movements of the animal, caused serious errors in observa- 

 tion, due to the displacement of the electrodes and their being thus brought near to or 

 in contact with surrounding structures. In the single case in which the 6th left 

 lumbar anterior root was divided and its central end connected with the galvanometer 

 electrodes, the difference (Cat) was found to be only '0045 Daniell. whereas the 6th 

 lumbar posterior root of the same side, when examined, showed a difference of 

 01 '2 Daniell. 



U. SPINAL CORD. 



The fact that the cord exhibits, like the nerve, both a persistent difference between 

 its longitudinal surface and its cross section and a true excitatory effect, was dis- 

 covered by DU BOIS-REYMOND, and has been since confirmed by other observers, notably 

 SETSCHENOW. The general features of the same were described by us in our preliminary 

 communication made to the Royal Society, but a systematic analysis of a number of 

 observations has never, that we are aware, been made, nor has the amount of the 

 persistent difference been determined. In the course of our investigations we have 

 noted the amount and characteristics of the resting difference in the spinal cords of 

 sixty-three Cats and fourteen Monkeys, and the results arranged in groups are given 

 in Appendix B. 



The most striking characteristics of the resting difference in the cord and the 

 contrast between this difference and that of the nerve may be roughly seen after the 

 death, of the animal (Cat, Rabbit, &c.) by rapidly exposing a length of cord and of 



