ON THE MAMMALIAN NERVOUS SYSTEM. 309 



changes and those indirectly produced by the reflex discharge of the cortex, ;is 

 described in the preceding chapters. To avoid this and to obtain uncomplicated 

 effects the cord was always severed from its connection with the higher centres. 



The plan followed in the experiment was, therefore, as follows : two situations 

 were selected, in almost all cases about the level of the 8th dorsal and 2nd to 

 3rd lumbar vertebrae ; the cord was then first exposed for a short distance at the 

 upper one and divided. It was then carefully exposed in the lower region and 

 divided again, the upper division being made first in order to diminish, by cutting 

 off all connection with the higher centres, the shock which subsequent operations 

 necessarily involved. The portion of cord (see fig. 10), included between the upper 

 or 8th dorsal section and the lower or 2nd lumbar one, formed thus an isolated 

 fragment, and since it was upon this portion that the experiments to be detailed 

 were carried out, this fragment may be termed the &quot; experimental region &quot; of the 

 cord. A portion of this experimental region was always prepared for observation at 

 either its dorsal or lumbar end, the preparation consisting in the exposure of from 

 4 to 5 centims. of the cord in immediate connection with the cross section and the 

 division of all root and other attachments. The cut end of the portion was then 

 ligatured, and the prepared part raised from the canal and suspended in air, care 

 being taken to avoid all undue pull upon the structure. As previously described, 

 (see Plate 29) the trunk was immovably fixed by holding the vertebrae in the ivory 

 jaws of a clamp, which was rigidly attached to a metal rod fixed into the experi 

 mental table. 



The ligatured end of the cord thus freed was attached by the before-mentioned 

 cables of thread soaked in 6 per cent. Nad solution and plastered with kaolin, to the 

 galvanometric non-polarisable electrodes, one cable being tied round the ligatured end 

 and cross section, the other round the longitudinal surface about 1 centim. distant. 



The usual resting difference between the surface and cross section was found to 

 be present in all cases, and was always kept carefully compensated ; its characteristics 

 have been fully gone into in Chapter IV., and it will therefore not be further alluded 

 to here. 



If the other exposed end of the experimental tract were now excited, either by 

 thrusting through it a pair of needle electrodes, or by placing upon the cut end the 

 points of a pair of ordinary platinum electrodes, an excitatory electrical effect was 

 seen in the portion connected with the galvanometric electrodes. This effect is 

 always of such a character as to be opposed to the resting difference ; it is suddenly 

 developed with the commencement of the stirmilation, and subsides on its cessation, 

 being followed by the rise in the resting difference which is described in Chapter IV. 

 It is evidenced both in the galvanometer and the capillary electrometer, the amount 

 of deflection of the needle and of movement of the mercury being dependent upon 

 (1) the nature, intensity, and number of the successive stimuli, (2) the condition of 

 the animal. 



MDcrcxoi. B. 3 B 



