ON THE MAMMALIAN NERVOUS SYSTEM. 487 



attachment to the cord, and that there are no fibres which pass directly from the 

 columns of the cord (above) outwards into the anterior roots without interruption in a 

 nerve centre. If any uninterrupted fibres exist, then the excitation of the central 

 end of an anterior root ought to give an effect in the galvanometer as arranged in 

 this experiment, for it is well known that the excitatory state (i.e., negative variation) 

 travels indifferently in either direction along continuous tracts. The absence of such 

 effect is a very convincing proof that such through fibres do not exist. Moreover, an 

 additional point receives elucidation from this same experiment. We have already 

 drawn attention to the fact that the anatomical relations of the pyramidal tract 

 fibres are for the most part (and wholly as far as the lower limb is concerned) not 

 with the efferent side of the spinal centres but rather with either the field of con- 

 junction or the afferent aspect. 



This anatomical juxtaposition of parts suggests to us that the pyramidal tract fibres 

 for the lower limb do not immediately end in the efferent corpuscles. Such a con- 

 clusion is in harmony with the experimental results now under discussion, for it is 

 unreasonable to suppose that the excitatory process would be so entirely blocked 

 when proceeding in the reverse direction along a path consisting merely of two fibres 

 (that of the anterior root and that of the pyramidal tract) joined by a large corpuscle 

 such as one of those in the ventral (anterior) horn. 



But we must not anticipate the facts which bear upon the mode of termination 

 of the fibres of the pyramidal tract, and must return to the question of the block 

 offered by the construction of a centre to the passage of nerve impulses. 



(2.) Resistance by the Afferent Side of a Nerve Centre to the Passage of Impulses. 



After having discovered the fundamental position that it is impossible for an excita- 

 tory condition to pass " backwards " through a nerve centre from the efferent to the 

 afferent side (whence it could easily spread up the cord), it was clear to us that the 

 next point to be brought into relation with that just described would be the possibility 

 of a nerve centre discharging its energy backwards down an afferent path ; or in 

 other words, whether the excitatory condition, if started in the afferent side of a nerve 

 centre, could be transmitted backwards along the posterior roots. Upon this subject 

 we have accumulated a number of facts and experiments from several points of view, 

 and as these are in mutual agreement we can answer the question unhesitatingly in 

 the affirmative. 



Many of the facts which establish the conclusion that a spinal nerve centre can and 

 does discharge energy down the posterior as well as the anterior roots when it is 

 stimulated into activity have more important bearings on other questions raised in the 

 present paper, and consequently have been referred to in Chapters VIII. and X. We 

 will, therefore, simply enumerate the facts, and refer the reader to these chapters in 

 which they are described, detailing only those which have not been already treated. 



