THE SPINAL CORD AS A CONDUCTOR 403 



The posterior spinal (sensory) roots at their entrance into the 

 cord divide into two bundles. The smaller of the two, situated more 

 laterally and consisting of fine fibres, enters opposite the tip of the 

 posterior horn and turns up at once in Lissauer's tract, a bundle of 

 fine longitudinal fibres close to the periphery of the cord. The fibres 

 seem to pass into and end in the substance of Rolando. The larger 

 median bundle of coarse fibres passes into the postero -external 

 column. Here each fibre divides into a descending and an ascending 

 branch, the former running in the comma tract, the latter in the poste- 

 rior columns up as far as the gracile and cuneate nuclei of the medulla. 

 Both of these branches give off collaterals in the whole of their course, 

 most numerous near the point of entry of the nerve. These collaterals 

 may be divided into four sets according to their destination : 



(1) Fibres ending round cells of anterior horn on same side or 

 crossing by posterior commissure to grey matter on other side. 



(2) Fibres ending in grey matter of posterior horns. 



(3) Fibres ending round cells of Clarke's column. 



(4) Fibres to lateral horn. 



Since the motor nerves arise from the anterior horn-cells, the 

 first set, the ' sensori-motor ' collaterals, represent the shortest possible 

 spinal reflex path. The second group may also represent a spinal 

 reflex path with two relays of cells, and therefore greater choice of 

 response and longer reaction time. The third set puts into action the 

 cerebellar tracts which arise from the cells of Clarke's column, and 

 therefore call into play a much more complicated mechanism, the 

 limits of whose action it would be difficult to define. The collaterals 

 to the lateral horn probably represent the afferent tracts of the 

 various visceral and vaso-motor reflexes which we shall study later. 



In dealing with the reflexes involving the co-operation of the 

 brain, we find no special tracts devoted to those impulses which 

 affect consciousness as sensations. All tracts going towards the 

 cerebral hemispheres are interrupted by cell relays, in the medulla or 

 cerebellum, and must serve as afferent channels for unconscious as 

 well as for conscious reactions. The quality of an afferent impulse 

 can only be defined by its origin, or by its effect on consciousness, 

 and much discussion has arisen as to the exact path of the various 

 cutaneous and muscular sensations in the cord. 



It is evident that an impulse may travel to the cortex by way of 

 the two cerebellar tracts through the cerebellum, or by way of the 

 posterior columns through the intermediation of the bulbar nuclei, or 

 by a series of relays from one segment of the cord to another through 

 grey and white matter alternately. It is supposed that all of the 

 ascending tracts may convey afferent impulses from the postericr 

 spinal roots to the brain, although evidence as to the part taken by 



