CHAP, i.] THE SPINAL COED. 895 



posterior column, reaching some little distance up, and a more 

 limited degeneration of a part of the median posterior column 

 stretching right up to the bulb. The position and form of the 

 tract of the degeneration in the median posterior column will 

 depend on the level of the lesion along the length of the cord, 

 according as it interrupts the ascending representatives of the 

 sacral nerves only, or of the lumbar and sacral nerves, or of the 

 dorsal and cervical nerves as well. A complete section or hemi- 

 section of the cord will produce results corresponding to the division 

 on both sides or on one side of all the nerves below the section. 



We may add that while, according to some observers, the 

 strand of fibres belonging to a particular root or group of roots, 

 having once taken up its position in the median posterior column 

 remains unchanged until it reaches the bulb ; according to others 

 it diminishes in area, some of its fibres making connections in the 

 cord itself. 



578. The antero-lateral ascending tract (Fig. 104, asc. a.L 

 is less well known than either of the two preceding ; it is also 

 more diffuse, that is to say, the fibres undergoing degeneration are 

 more largely mixed with fibres of a different nature and origin. 

 It appears to extend down the cord to a lower level than the 

 cerebellar tract, but its lower limit has not yet been accurately 

 determined. Since the degeneration taking place in it is an 

 ascending one, it has been inferred that it serves as the path 

 for afferent, and indeed for sensory impulses. Degeneration in 

 it is seen only after section or injury of the substance of the cord 

 itself, not after division of the posterior roots. If, then, it is to 

 be regarded as a channel of afferent impulses passing into it 

 from the posterior roots, those impulses must pass into it along 

 those fibres of the posterior root which find secondary trophic 

 centres in some part of the grey matter ; in this respect this tract 

 resembles the cerebellar tract, and differs from the median 

 posterior tract. The latter is the direct continuation up the cord 

 to the bulb of such fibres as are still trusting for their nutritive 

 activity to the cells of the ganglion on the posterior root; the 

 fibres of both the former trust for their nutritive activity to some 

 part of the grey matter of the cord, and presumably to the nerve- 

 cells of that grey matter. A further resemblance between the 

 antero-lateral ascending and cerebellar tracts must be admitted, if 

 future researches confirm the opinion of those who hold that the 

 former like the latter, at the top of the cord, pass along the 

 restiform body to the cerebellum. Indeed under such a view it 

 would appear probable that the antero-lateral tract is simply a 

 more diffuse and outlying part of the cerebellar tract. 



579. We may now briefly pass in review, somewhat as 

 follows, the chief facts which we have learnt concerning the 

 structure of the spinal cord, always keeping in view their physio- 

 logical meaning. 



F. 57 



