ON THE MAMMALIAN NERVOUS SYSTEM. 307 



We have noticed that it is more pronounced when the etherisation has become 

 slight in degree. 



In addition to the after-effect due to excitation there are seen under certain circum 

 stances, particularly that of inadequate etherisation, changes in the nerve difference, 

 which in the galvanometer indicate their presence by the needle now slowly rising 

 30 to 50 scale, and now falling. These disappear when the anaesthesia is rendered 

 more profound, and are much more marked in the case of the spinal cord ; they are 

 probably due to the occasional discharge of groups of minimal nerve impulses, and are 

 analogous to those first observed in the medulla of the Frog by SETSCHENOW. 



B. POSTERIOR ROOT. 

 1. Amount of Difference. 



We have made six experiments upon the posterior root. In these a lumbar posterior 

 root was exposed, ligatured, and divided as close to the ganglion as possible, without 

 involving this structure, and the root thus left connected with the cord was svispended 

 in air by the attached thread. The particular root chosen in five cases was the 7th of 

 the lumbar series, since this is the largest in the Cat of the roots forming the lumbar 

 plexus (see Plate 35). The galvanometer electrodes were attached, as in the nerve, by 

 means of thread cables moistened with 6 per cent. NaCl, to the cross section and the 

 longitudinal surface, and the amount of the resting difference determined by the 

 balancing method. 



The result in the five animals was found to be 026, &quot;02, 02, &quot;018, and 016 Daniell 

 respectively. 



Hence although the 7th lumbar root is less than half the size of the sciatic nerve, 

 the average amount of the resting difference, &quot;02, is about twice as large. (The Gth 

 lumbar root was observed once, the difference being &quot;012.) This remarkable circum 

 stance is very significant when the proximity of the structure to the spinal cord is 

 taken into consideration, and deserves a more extended investigation. It may be 

 pointed out that SCHIFF observed that there was a resting difference between contacts 

 when placed upon a more central and a more distal portion of a continuous nerve, the 

 central contact being positive to the other ; also that excitation of any portion of the 

 tract caused an excitatory change, in which the tissue under the first contact became 

 galvanometrically negative to that under the second more remote one. 



Further, both GRUNHAGEN and BERNSTEIN have noticed that the central portions 

 of the nerve are more excitable than the distal portions. Now whether the persistent 

 difference be fundamentally the same in kind as the transient excitatory electrical 

 change or not, there is an undoubted quantitative relationship subsisting between 

 the two. It is, therefore, not surprising that there should be found an increase 

 in the amount of the resting difference obtainable in the posterior roots as compared 

 with the sciatic nerve. 



2 K 2 



