300 MESSRS. F. GOTCH AND V. HORSLEY 



the application of the electrodes. Fatigue of the cortex was avoided by suitable 

 intervals of rest. 



The intensity and character of the stimulus is referred to under apparatus, p. 291, 

 but it may be here stated that the electrodes were fine platinum points 1 to 2 mm. 

 apart. 



B. Corona Radiata. To expose the corona radiata it was necessary to raise the 

 cortex by a horizontal incision, and then to check the bleeding from the pial vessels 

 with pieces of amadou around the cut border of the area exposed. If after several 

 excitations the exposure had lowered the excitability of the fibres, the electrode points 

 were sometimes inserted 1 mm. deep into the substance of the corona, instead of, as 

 usual, resting gently on the surface. The electrodes employed were the same as those 

 for the cortex. 



C. Spinal Cord. The excitation of the spinal cord, especially for purposes of 

 differentiation of the columns naturally demanded special attention. We obtained 

 our initial generalisations by means of exciting needles fixed on either side of the 

 cord or pressed against the longitudinal columns, vide pp. 369, &c. 



This method we soon discarded (Section 6, Part V.,) for that now to be detailed, 

 viz., the excitation of the cut sections of the columns of the cord, inasmuch as the 

 differentiation of the bundles of fibres could thus be accomplished at the same time as 

 other objects in view. The cord having been previously divided in the manner stated 

 above, and the surface and surrounding tissues being carefully dried, the fine platinum 

 electrodes, 1 mm. apart were used. It was our aim to provide for the excitation of 

 as many fibres in any column as possible in order that the largest galvanometric 

 effect might be evoked, and at the same time to avoid extrapolar excitation of 

 neighbouring columns. These particulars were fulfilled by applying the two electrode 

 points, as indicated on the accompanying fig. 2, i.e., vertically on the anterior and 

 posterior columns, and horizontally on the lateral column in the region of the crossed 

 pyramidal tract. 



Fig. 2. 



Photograph of a transverse section of the fresh spinal cord (Cat) at the 7th dorsal nerve, slightly 

 magnified. The remarkable differentiation of the posterior columns in this animal (SciiiFr) is 

 well seen. 



That these measures were effectual in localising the stimulus was demonstrated by 

 the fact that the placing of the electrodes on the neighbouring sections of the grey 

 columns evoked no electrical changes, as evidenced in the galvanometer connected 

 either with the issuing- nerves or with another portion of the cord ; so that the 



