ON THE MAMMALIAN NERVOUS SYSTEM. 391 



both sides. This, therefore, suggests that descending impulses in the lateral column 

 are, to a very large extent, confined to these fibres, but those in the posterior column 

 cross into similarly situated fibres on the opposite half of the cord. 



A second experiment of the reverse kind was made on another occasion, the excita- 

 tion being now in the lumbar region, and the impulses evoked thus being ascending. 

 The cord was divided at the 7th dorsal and 3rd lumbar, and the upper dorsal portion 

 was prepared for galvanometric observation, and split longitudinally, whilst the 

 lower lumbar portion was prepared for excitation ; the general arrangement is shown 

 in fig. 17. 



When the right half of the split cord was examined 



Excitation of the right lateral produced an effect of 38 

 left 3 



,, right posterior ,, 40 



,, left ,, ,, ; , 14 



The cord was then again divided a little centrally to the seat of the previous 

 excitation, so as to expose a fresh surface for the stimulation, and now 



Excitation of right lateral produced effects of 86 ; 94 



left ., ,. 16 15 



,, right posterior ,, ,, 81 112 



left 52 60 



The left half of the cord when observed gave the following results : 



Excitation of left lateral produced effect of 33 

 right ,, ,, ,, 3 



left posterior ,, ,, 40 



right ,, ,, 7 



The general conclusion to be derived from these experiments is that the excitation 

 of the lateral column produces an effect which is limited to a very remarkable degree 

 to the side excited. On the other hand, the excitation of one posterior column 

 produces with appropriate excitation bilateral electrical effects, but the effect is twice 

 as great on the same side as it is on the opposite one. 



It must, however, .be borne in mind that the operation may, by depressing the 

 excitability of the posterior columns, be a source of error which can interfere largely 

 with the above results, since the posterior columns are more liable than the lateral to 

 suffer by the procedure of splitting, and earlier experiments have convinced us that if 

 they are injured the electrical change produced by their excitation is very largely 

 affected. 



The extent to which the effects and thus the nerve impulses are localised in the 



