490 MESSRS. P. GOTCH AND V. HORSLEY 



are given in Chapter X. Reference to this Chapter will show that the effects in the 

 posterior roots may be due to (a) conduction of nerve impulses from the excited area 

 down directly continuous fibres, (6) conduction of nerve impulses down fibres which 

 come into relation with cells, and only indirectly with the fibres in the roots. To 

 these two must now be added (c) the discharge of impulses from the centres in the 

 cord aroused by the excitation. 



It is evident that the discrimination of these different factors can only be achieved 

 by careful comparative experiments made upon the cord, when its direct connections 

 with the roots haying been interrupted, the centres ai'e placed under different 

 conditions as regards excitability. These experiments have not as yet been suffi- 

 ciently satisfactory to enable us to form any decisive opinion on this point. 



There is. however, one very important fact already referred to at the close of the 

 preceding Chapter X., to which we must now draw attention. 



It will be remembered that, on comparing the results of the experiments detailed 

 in Chapters IX. and X. respectively, the inference was suggested that the indirect 

 channels by which the cord communicates with the posterior roots, are of such a 

 character that they offer more resistance to the passages of impulses from the cord to 

 the nerve than to the passage of similarly evoked impulses from the nerve to the 

 cord. There is thus evidence that, as regards these indirect channels, the same 

 phenomenon of unequal conduction is present which, in its most marked form, 

 exists in the case of the efferent region of the spinal centre. 



It must, however, be borne in mind that these indirect channels do not necessarily 

 form the afferent side of the centres, but are only in relation with them, since it is 

 quite possible that in many instances these fibres pass through cells without 

 entering that field of conjunction which forms the terra incognita of the centre. 

 Evidence, therefore, of the nature of the discharge from this terra incognita backwards 

 down the posterior roots, as well as forwards through the anterior cornual cells, could 

 only be obtained through electrical excitation of the cord, if such excitation was 

 limited to the pyramidal tract. The simplest way of performing this experiment is 

 that of exciting the commencement of this tract in the cortex cerebri. and of 

 observing the changes in the posterior roots. We have endeavoured in two animals 

 to perform this experiment, but without obtaining as yet any satisfactory evidence, 

 nor is this to be wondered at, when it is borne in mind how comparatively small the 

 effects are in the mixed nerve under these conditions. 



Summing up the evidence obtained from all methods of experimentation, we are led 

 to conclude that when a spinal nerve centre is thrown into activity, a portion of its 

 energy flows as a discharge backwards down the posterior roots as well as forwards 

 down the efferent fibres of the anterior roots, and upwards and outwards along 

 internuncial fibres to the next centres. 



With this somewhat enlarged view of what occurs when a spinal nerve centre 



