ON THE MAMMALIAN NERVOUS SYSTEM. 



403 



presumably due to nerve impulses proceeding up channels which are composed of fibres 

 starting below the section in the posterior column, but brought, by the intervention of 

 cells, into relation with fibres contained in the lateral column of the same side, and in 

 the posterior of the opposite side. 



An instructive experiment bearing upon this subject remains to be detailed. We 

 divided the posterior roots of the 5th, 6th, 7th lumbar, and 1st and 2nd sacral 

 nerves on the left side. After twenty-one days the animal (Cat 227)* was used for 

 experiments on the cord. These were of the same kind as the previous ones, the 

 cord being divided in the dorsal and lumbar regions, and the experimental tract 

 prepared for attachment to the galvanometric electrodes at its dorsal end and for 

 stimulation at its lumbar. The excitatory electrical effects obtained with excitation 

 of the different columns are given in the following table. 



DIVISION of Left Posterior Roots of Sciatic Nerve. 



Average effect obtained by exciting both lateral columns 106 



Average effect obtained by exciting posterior column on side of lesion 52 

 Average effect obtained by exciting posterior column on opposite side 



to lesion .114 



It is clear, therefore, that the result of the division of the root was to reduce the 

 posterior column effect on the side of the lesion, the diminution of effect amounting 

 to about a third of what must be considered its full value, since the posterior column 

 effect on the side opposite the lesion is probably lower than it would be in the normal 

 animal. 



If, therefore, the stimulus had been of sufficient intensity to evoke impulses trans- 

 mitted by direct fibres only, then this diminution would mean that a third of these 

 direct fibres had degenerated in consequence of the section. The stimulus, however, 

 was evidently too strong to be classed as " minimal," and all the effects must be con- 

 sidered as increased by the accession of impulses proceeding up indirect paths. 

 Hence, the experimental results suggest that from the total number of direct and 

 indirect fibres uniting the lumbar region of the left posterior column with the dorsal 



* For full description of this animal see p. 466. 

 3 F 2 



