ON THE MAMMALIAN NERVOUS SYSTEM. 277 



are further known to gradually tend towards the middle line of the cord as they are 

 displaced by others entering the cord in proceeding upwards. Further these direct 

 fibres are strictly unilateral, so far as is known at the present time. 



There are, in addition, also other known systems of ascending channels, but these 

 are not directly continuous with the root, but start indirectly from central mechanisms 

 which, to judge from the degeneration method, although this is of course not absolute, 

 appear to intervene. These are (l) the direct cerebellar tract which runs up the 

 posterior and outer surface of the lateral column, and (2) the antero-lateral tract 

 which occupies a similarly lateral position further forwards on the margin of the cord. 



In addition to the foregoing, there is some evidence to show that internuncial fibres 

 run in the lateral column in about the inner third of its centre, or a little posteriorly 

 to this point. Of the existence of channels for transmission of impulses upwards in 

 the opposite posterior column to the side of the root by which they enter the cord 

 nothing is known for certain. 



History of the Oalvanomc.tric Method of Determining the Action of the Nerve 

 Centres and the Course of Nerve Channels. 



As we have before frequently acknowledged, the real basis of the galvanometric 

 method was the discovery by DU BOIS-REYMOND* of the negative variation produced by 

 excitation in the resting electrical difference of a nerve path. Since that discovery, 

 the idea has doubtless occurred to various physiologists that, by this means, we might 

 discover the mode of functional activity of nerve centres. CATON! was the first to our 

 knowledge who directly employed the galvanometric method of determining such 

 variations for the investigation of the activity of nerve centres, and the localisation 

 of the same. He connected points on the external surface of the cortex with the 

 galvanometer, and he found that the uninjured external surface of the brain was 

 usually positive to a section of the same. When any part of the cortex thus 

 investigated was thrown into activity, the resting difference showed distinct negative 

 variations. Thus, in the Monkey, after the cortex had been prepared and the 

 electrodes applied to the centres (FERRIER) of rotation of the head and mastication, 

 the negative variation or action current showed itself when these movements were 

 performed. Further, in the Rabbit, when the area of the cortex which subserves the 

 movements of the eyelids was investigated, the negative variations showed themselves 

 when the opposite retina was illuminated. SETSCHENOW]: was the first to our know- 

 ledge who connected the medulla oblongata with the galvanometer. He noticed 

 certain periodic variations in the resting electrical difference, which he attributed to 

 periodic changes of the functional activity in the bulbar centres. 



* ' Untersuchungen Uber Thierische Elektricitat.' 



t ' British MedicalJournal,' 1875 ; also ' Transactions of the IX. International Medical Congress, 1887.' 



J ' Archiv fiir gesam. Physiologie,' PFLUOEB, vol. 25, 1881, p. 281. 



