ON THE MAMMALIAN NERVOUS SYSTEM. 373 



In our earlier experiments we made use of the capillary electrometer, and we will, 

 therefore, first briefly describe those made with this instrument. 



SECTION 4. EXCITATORY ELECTRICAL EFFECTS IN THE CORD INVESTIGATED BY 

 MEANS OF THE CAPILLARY ELECTROMETER. 



The electrometer, owing to the rapidity with which the mercury moves, furnishes 

 valuable information as to the alterations in character and amount of electrical 

 changes which follow one another in rapid succession. We have already indicated 

 the experimental advantages which this confers in the examination of the cord effects 

 evoked by cortical stimulation.* Since, however, the conditions which increase the 

 sensitive characters of the instrument are to a great extent those which diminish its 

 rapidity, it is almost impossible at present to obtain an instrument of sufficient 

 sensibility for our purpose, without so slowing its movement that it takes more than 

 Yg- second for the mercury to complete its rise or fall. If, therefore, a series of transient 

 electrical changes similar in direction, following one another at intervals of less than 

 ~YJ second and all of equal intensity, are allowed to affect the instrument, then since the 

 movement of the meniscus due to the first change would take -^ second for its comple- 

 tion, the second change will occur when the mercury is either in movement or has just 

 completed its excursion and not returned ; a second movement is thus super-imposed 

 on the first ; so a third on the second, until a limit of fusion is reached, this being 

 dependent upon the fact that with each additional displacement the counter-pull of 

 the surface tension increases, and finally the displaced mercury is maintained at a 

 new level without any additional displacement perceptible on either side of the level 

 attained. It is probable that an extremely fine vibration synchronous with the rate of 

 successive electrical effects exists, but when the successive electromotive changes are 

 uniform in amount, direction, and time relations, such a vibration must be extremely 

 small. 



A very different condition is introduced when these electrical changes are alternate 

 in direction, since now each displacement by one effect, whilst still in progress, is 

 counteracted by another in the reverse direction due to the succeeding change being of 

 opposite sign to its predecessor. The effect of such a series of electrical changes even 

 when following one another at such short intervals as T J^ second is thus clearly visible, 

 whatever level the mercury may have reached, as a blurring of the edge of the 

 meniscus. The appearance to the eye may be described as a grey border to the 

 otherwise black opaque column when viewed under the microscope. 



This peculiarity of the electrometer at once enables us to judge whether, in a series 

 of brief electrical changes, these are similar or dissimilar in direction, and we will 

 first draw attention to this point in connection with the electrical effects in the 

 spinal cord. 



* See this paper, p. 324, also ' Boy. Soc. Proc.,' Nov., 1888 (vol. 45, p. 18). 



