ON THE MAMMALIAN NERVOUS SYSTEM. 449 



connection in some way with some element in the cord, which serves as a common 

 centre both for nutrition and for growth. 



The researches on the conduction of afferent impulses from the nerves into the cord 

 detailed in the preceding chapter, as well as our other experimental investigations, 

 show that such paths undoubtedly exist. The present experiments will prove that 

 these, although indirect, are capable of conducting impulses from the centre towards 

 the periphery ; the only distinction between the indirect and the direct path, in this 

 respect, being the greater intensity of the stimulus necessary to produce the evidence 

 of such effects in the case of the former, and the comparatively small amount of the 

 nerve energy which can be thus evoked, through the indirect path, in the afferent 

 nerves. In other words, the direct path is that of least resistance to impulses when 

 these pass backwards from the cord into the afferent nerves. 



We now pass to the consideration of the detailed plan of the experiments as a 

 necessary prelude to the analysis of the results. 



SECTION 2. PLAN OF EXPERIMENTS AND PRELIMINARY OBSERVATIONS. 



Our first experiments upon the nerve effects following cord excitation were crude in 

 design. They were made by first dividing the cord in the dorsal region, then 

 preparing one sciatic nerve in the back of the thigh, ligaturing it, dividing it, and 

 connecting its central end and the adjoining longitudinal surface by means of cable 

 electrodes with the galvanometer, and finally stimulating the cord by means of 

 needles which pierced it and acted as electrodes to the secondary coil of the 

 inductorium. We satisfied ourselves in this way that the excitation of the cord was 

 followed by excitatory electrical changes in the nerve, and then proceeded to more 

 methodical experiments. In these the cord was carefully exposed by dissection and 

 a piece removed, so that the cross section of its surface could be easily seen. The 

 various cut ends of the columns were then excited as desired by a series of interrupted 

 induction currents (Helmholtz side-wire) for a period which was controlled by the 

 revolving mercurial key, and was carefully kept of the same duration during any set 

 of observations. A pair of well-insulated fine platinum-pointed electrodes were used 

 for the stimulating current, these being applied to the particular region of the cross 

 section it was desired to excite, in the manner and under all the precautions already 

 described in Chapters III. and VIII. The sciatic nerve when raised in air and con- 

 nected with the galvanometer displayed the usual resting electromotive difference 

 between its two points of contact. This difference has been referred to at length in 

 Chapter IV. and was compensated in all cases. On exciting the cord for five 

 seconds an electrical effect was produced in the nerve which was always opposed 

 in direction to the resting current, and which passed away on the cessation of the 

 stimulus. The amount of the deflection was in most cases considerably less than 

 was obtained by applying the same stimulus (unaltered in intensity and duration) 



MDCCCXCT. B. 3 M 



