ON THE MAMMALIAN NERVOUS SYSTEM. 489 



The employment of clamps, &c., obviated any errors, due to the movement of the 

 animal ; the experiment may, however, be objected to on the ground that the 

 strychnia discharges are abnormally intense, and must thus break through resistances 

 which ordinary central discharges would be unable to. That the centres can dis- 

 charge down posterior roots without being under the influence of strychnia is, how- 

 ever, shown by the frequent occurrence of similar though smaller galvanometric 

 effects in the posterior root of an unstrychnised animal, when the cord is discharging 

 in consequence of previous electrical excitation and shallow narcosis. 



(2.) The Electrical Changes in the Posterior Roots when the Spinal Centres are 

 Discharged Rcflexly. 



As in the preceding experiments, so here, the evidence is that derived from the 

 electrical changes observed, (a) when the posterior root was directly connected with 

 the galvanometer, and (b) when the (mixed) sciatic nerve was connected (after 

 division of all anterior roots) with the galvanometer and the central eud of one 

 posterior root excited. 



(a.) In this experiment the central end of one divided posterior root was connected 

 with the galvanometer and the sciatic nerve of the same side stimulated, the result 

 being to produce in the galvanometer a deflection of 20 scale whenever the muscles 

 were thrown into a fair reflex spasm (coil 8000, 341). We shall see presently that this 

 is the amount frequently obtained from the mixed nerve as the result of a reflex dis- 

 charge. It is most interesting, therefore, to see that the centre reflexly discharges 

 backwards down the posterior root. The bearing of this on the discussion as to 

 which part of a centre is probably the source of the kinetic nerve will be seen 

 further on when this question is raised. 



(b.) We varied the experiment in another animal, Cat (268), by dividing all the 

 anterior roots supplying the left sciatic nerve and connecting the cut central end of 

 this latter with the galvanometer. We then divided one posterior root, the 6th left 

 lumbar, and excited its central end. 



By this arrangement we obtained excitation of the spinal centres in the lumbar 

 enlargement, and at the same time left only the posterior roots or afferent channels 

 for any downward discharge such as would evoke electrical changes in the sciatic 

 nerve. 



One observation gave an effect of 10, another of 19 in the galvanometer, the reflex 

 being weak in the first and fairly strong in the second. 



(3.) Existence of Electrical Changes in the Posterior Roots ivhen the Spinal Centres are 

 aroused by Electrical Excitation of the Columns of the Cord. 



The details of these experiments, in which a posterior root was connected directly with 

 the galvanometer and the cut surface of the spinal cord in the dorsal region excited, 



MDCCCXOI. B. 3 R 



