1888.] The Spinal Cord and the Cortex Cerebri. 25 



EXPLANATION OF PLATE 1. 



The six figures in the plate are facsimile drawings of photographs. The negatives 

 were obtained by projecting the image of the capillary electrometer upon a narrow 

 slit, behind which an extra rapid photographic plate travelled. The direction of 

 movement was such that the right hand side of the prints corresponds to the moment 

 when the plate reached the slit : the figures are thus to be read from right to 

 left. In order to save room, only the essential part of the photographs that 

 showing the position of the meniscus of the mercury in the photograph is shown. 

 The lower darkly toned part of each figure corresponds to the lighter part of the 

 negative, and indicates the part of the slit shaded by the mercury of the electro- 

 meter ; an excursion of the mercury is thus indicated by an elevation or depression 

 of the upper edge of the dark band. The regular series of dark and light bars 

 on the edge of the figures were made by a vibrating shutter, each entire vibration 

 of which occupied one-tenth of a second. 



FIG. 1. Photograph showing two prominences, m and b, due to two excursions of 

 the mercury when first a make and then a break induction shock was 

 led through the mammalian nerve, the cut end and surface of which were 

 in connexion with the electrometer 6 cm. from the point of excitation. 

 The arrows indicate the direction of the exciting induction current through 

 the nerve, and the effect is seen to be independent of this direction. At 

 the point marked * the electrometer was short circuited, and the move- 

 ment of the mercury due to the cessation of the demarcation current effect 

 is thus shown. The excursions at m and b are seen to be opposed in direc- 

 tion to that produced by the demarcation current. 



FlG. 2. Photograph showing the excitatory variation effect in nerve. In this case 

 the nerve of the monkey was severed from the body, connected as in fig. 1 

 with the electrometer, and excited six times by means of induction shocks 

 of different character and direction. The excitation occurred at make / 

 and break b, and the direction of the induction shock whether t, ascending, 

 or \, descending is indicated. The effect is seen to be always in the same 

 direction, being opposed to that of the demarcation current, and such that 

 the electrode on the longitudinal surface becomes negative to that on the 

 cut section. The rate of movement of plate was the same as in fig. 1. 

 FlG. 3. Photograph illustrating the effect produced in the electrometer when 

 there is a slight escape from the exciting electrodes into the electrometer 

 electrodes. The effect was produced by using a severed nerve, which no 

 longer gave any obvious excitatory response to electrical excitation. The 

 exciting electrodes were placed upon such a nerve at a very short distance 

 (1'5 cm.) from the nearest leading off electrode, viz., that upon the longi- 

 tudinal surface. The direction of the effect is seen to depend upon the 

 direction of the induction shock as produced by make m and break b of the 

 primary circuit of the induction apparatus. The character of the excursion 

 is markedly different to that shown in figs. 1 and 2, being much more 

 abrupt. The rate of movement of plate was the same as in 6g. 1. 

 FIG. 4. Photograph showing the effect produced in the electrometer when this 

 is connected by one pole with the longitudinal, and by the other with the 

 sectional, surface of the spinal cord of the monkey, and the cortex cerebri 

 then excited over the motor area for the lower limbs by means of the 

 faradic current. The excitation commenced at a and ceased at c. It is 

 seen to be accompanied by an upward movement of the mercury, shown by 

 an alteration in the position of the dark band, which reaches a slightly 



