THE SPINAL CORD AND REFLEX ACTIONS. 599 



which do not degenerate. It resembles the cerebellar tract 

 and differs from the median posterior in only undergoing de- 

 generation after section of the cord itself, and not of the pos- 

 terior roots also. The upward ending of its fibres is still 

 uncertain, but is probably in the cerebellum: the origin of the 

 fibres is in the gray matter of the cord. 



Allowing for all the tracts of degeneration above described 

 it will be seen that considerable portions of the white col- 

 umns of the cord (left unshaded in Fig. 175) are un- 

 affected: at the most, trifling degenerations extending a little 

 way above or below the point of cross-section are found. 

 Some of these are due to bundles of posterior root-fibres which 

 run for a short distance in the external posterior column, epc, 

 before separating into two sets, one entering the gray matter 

 of the posterior cornu, the other passing into the internal 

 median tract. Another bundle of posterior root-fibres runs 

 up in the cord a short way in what is named the column of 

 Lissauer, It, and gives rise to an ascending degeneration ex- 

 tending a short way above any hemi section. The main bulk 

 of the unshaded parts of the figure, however, represents longi- 

 tudinal fibres which do not degenerate up or down after section 

 of the cord : they appear, therefore, to have nutritive centres 

 at each end ; and probably are fibres uniting different levels 

 of the gray matter. In addition to the longitudinal are of 

 course some horizontal : these are partly fibres of spinal roots 

 passing into the gray core, partly medullated fibres crossing 

 the middle line in the anterior white commissure; and in 

 addition to fibres of the gray matter proper uniting its halves 

 across the middle line are many fine medullated fibres which 

 run dorso-ventrally and from side to side in it. 



As regards longitudinal conduction in the white columns of 

 the cord, we may sum up the main facts as follows : Tho py- 

 ramidal and direct pyramidal tracts consist of efferent fibres 

 uniting the cerebral cortex with various levels of the gray 

 matter of the cord from which motor fibres for the voluntary 

 muscles pass out. The descending antero-lateral tract prob- 

 ably also contains efferent fibres uniting the brain with 

 different parts of the gray matter of the cord. The cerebellar 

 and ascending lateral tracts convey afferent impulses from the 

 gray matter of the cord to the brain, but are only indirectly 

 connected with the fibres of the dorsal spinal roots. The 

 median posterior tract is afferent and mainly made of fibres 



