ON THE MAMMALIAN NERVOUS SYSTEM. 427 



in order to avoid cooling, &c., and only taken out of its muscular bed for purposes of 

 excitation. The large resting electromotive difference between the surface and cross 

 section of the cord having been balanced, an excitatory electrical effect in the opposite 

 direction to the difference was observed to accompany the stimulation of the nerve. 

 The amount of this effect, as indicated by the amount of the galvanometric excursion, 

 varied, as previously stated, not merely with the intensity and the duration of the 

 stimulus, but with the condition of anaesthesia, &c., of the animal. If care be taken 

 to keep these factors as far as possible unchanged, the excitatory electrical effect thus 

 produced at each repetition of the experiment keeps very fairly uniform, as much so 

 in point of fact as in similar experiments upon Mammalian nerve trunks. When we 

 had thus obtained a constant effect in the cord, an intervening section was made in 

 the lower dorsal or upper lumbar region by the method described in Chapter III., 

 Section 2. Its position coincided, in some cases, with the lowest portion of the part 

 of the dorsal cord exposed for connection with the galvanometric electrodes, in the 

 majority of cases with a still lower level obtained by a fresh exposure of a small 

 portion of cord. It was found that it was not desirable to make this second exposure 

 when the cord was first prepared, since during the time lost in the preliminary experi- 

 ment, &c., the intervening coi'd at this point is apt to suffer. 



The intervening section having been thus made, the nerve was excited under 

 precisely the same conditions as before, and the alteration, if any, in the amount of 

 the cord effect noted. In all such experiments the extent of the section of the cord, 

 and thus the part involved in the interruption, is capable of great variation, and the 

 results may with advantage be grouped in relation to the particular regions which 

 have been involved in it, thus following the exact and strictly logical method which, 

 under LUDWIG'S guidance, was such a characteristic feature in WOROSCHILOFF'S 

 treatise. 



SECTION 6. THE INFLUENCE OF HEMISECTION. 



The results of a hemisection made between the portion investigated and the entry 

 of the stimulated nerve as regards its influence upon the electrical changes in tht 

 cord, which are evoked by the nerve stimulation, will be best seen by a glance 

 at the subjoined table, which gives the result of the comparative observations made in 

 the manner previously indicated in two animals before and after the section. The 

 animal was in all cases carefully ana3sthetised and kept well under the influence of 

 the anesthetic ; the stimulus was always in any two comparative results adjusted to 

 the same degree of intensity, and was of the same duration, whilst, as far as possible, 

 errors due to variations incidental to the application of the stimulus, the condition of 

 the animal, and the galvanometric condition of the observed region of the cord, were 

 excluded, and observations in which contemporaneous changes in either of these three 

 conditions could be detected were rejected. 



3i2 



