ON THE MAMMALIAN NERVOUS SYSTEM. 395 



be presumed to operate in these cases, though the results of the experimental observa- 

 tions set forth in the succeeding chapters all favour the supposition that the fibres 

 which form the indirect crossed path are almost entirely derived from the posterior 

 column, whilst the internuncial fibres, by which the successive segmental groups of cells 

 are associated with one another, are mainly confined to the lateral column. It will be 

 observed that the crossed effect is only j to ^ of the direct effect evoked by stimulation 

 of the side opposite to the lesion ; hence either the nerve impulses, of which the effect 

 is the index, are either greatly reduced in quantity by their passage through the grey 

 matter, or comparatively few nerve impulses take this crossed path. 



Finally, as regards the effects in the side opposite to the lesion, it will be noticed 

 that, comparing the relative value of the two effects, due to excitation of the 

 posterior column is now less than that of the lateral column. This is probably due 

 to the necessary exposure of the columns at the 12th dorsal vertebra acting injuriously 

 upon the excitability and conductivity of the exposed but otherwise uninjured 

 posterior column not included in the section. 



A glance at the full details of experiments 351 and 371 will show (see table, 

 Section 6 (&)), that the same relation was present in most cases before the hemisection 

 was made ; the subject will be referred to in more detail in the next paragraph, which 

 deals with the result of section and injury to the posterior columns. 



B. Influence of Section of Posterior Columns. 



The influence of the section of both posterior columns in the lower dorsal region 

 upon the electrical effect evoked in the lumbar region, and thus upon descending 

 nerve impulses, is shown by the following experiment ; 



The cord was divided in a Cat (357) at the 9th dorsal and at the 2nd lumbai 

 vertebras, it was prepared for excitation at the former region, for attachment to the 

 galvanometric electrodes at the latter. The cord was then also exposed at the llth 

 dorsal vertebra, and, before any operative lesion, an experiment of the customary kind 

 was made. The excitation, with stimulus 500 of the two laterals, gave deflections of 

 30 and 65, average, 47 ; that of the two posteriors gave 130 and 150, average, 140. 

 After division of both posterior columns at the 1 1th dorsal, similar excitation of the 

 lateral columns evoked effects of 60 and 105, average, 82,* whilst that of either 

 posterior column evoked no effect whatever ; unfortunately, the effects evoked by 

 stronger stimuli were not observed. 



The interruption, therefore, as regards the posterior columns was complete with 

 this intensity of stimulus, and hence all nerve impulses generated by such " minimal " 

 excitation of either column in the dorsal region are propagated to the lumbar region 



* It will be seen that the average effect obtained due to excitation of the lateral columns after section 

 of the posterior columns is increased. A similar exaltation occurs in the experiments detailed in 

 Chapter IX., Section 7, C, where the causation of the phenomenon is discussed. 



3 K 2 



