ON THE MAMMALIAN NERVOUS SYSTEM. 463 



The lateral column of the opposite side with no part of the effect. 



It is most remarkable how nearly these average figures resemble those obtained 

 by exactly similar average methods when the whole mixed nerve was the part 

 investigated. 



This similarity suggested to us the possibility that the fibres in the anterior 

 roots are but slightly concerned in the production of the electrical effect in the mixed 

 nerve when the cord is excited by such strengths of stimulus as have been used in the 

 foregoing experiments (500-2000, very rarely 4000). 



It will be found on referring to Chapter VI., that whereas the electrical effect 

 observed in the cord to follow excitation of the cortex is very considerable, it is very 

 small in the sciatic nerve, and we there suggest that this difference involves a change 

 in the amount, intensity, or quality of the nerve impulses in their passage through the 

 unknown endings of the pyramidal tracts, and the known origins of the efferent 

 nerves, the anterior cornual corpuscles. (See fig. 22, p. 495, Chapter XL) 



To what extent, and under what circumstances, electrical effects can be detected in 

 the sciatic nerve, when all the posterior roots are divided and the nerve is connected 

 with the cord by efferent fibres only in the anterior roots, becomes therefore a most 

 interesting question. 



To the consideration of experiments upon this point we will now turn. 



SECTION 5. THE ELECTRICAL EFFECTS IN EFFERENT NERVES FOLLOWING 

 EXCITATION OF THE SPINAL CORD. 



The least complicated mode of experimentation for determining these effects would 

 be that of directly observing the changes in the central end of a divided anterior 

 root. This experiment, however, we have not yet successfully accomplished ; the 

 difficulties in the way of obtaining satisfactory connections with a divided anterior 

 root are augmented by its anatomical relations, and by the fact that, in order, as we 

 shall see, to obtain any changes in the root an intensity of stimulus has to be applied 

 to the cord which evokes general movements, thus dragging on the short root. The 

 dangers of inadequate isolation have been already dwelt upon in Chapter IV. Our 

 attempts in this direction were so unsuccessful that we determined to employ the 

 more laborious method of division of all the posterior roots, and examination of the 

 electrical changes in the sciatic nerve. 



The plan of experiment, therefore, consisted in exposing the lumbar cord and 

 cauda equina (see Plates 34 and 35, and fig. 18), and then dividing the posterior roots 

 of the 4th, 5th, 6th, 7th lumbar, 1st and 2nd sacral nerves, so as to leave the sciatic 

 connected with the cord by the efferent fibres only. 



We made experiments upon seven animals (Cats) on these lines ; but in two of these 

 we unfortunately did not divide the 5th lumbar posterior root and the connection 



