22 THE SPINAL CORD. 



that both from the root-fibre before its bifurcation and from its ascending and 

 descending stems there are given off at tolerably freqnent intervals "collateral" 

 branches which are directed inwards towards the grey matter ; into which they 

 penetrate for a variable distance and within which they ultimately end by breaking 

 up into a ramification of nerve-fibrils which may frequently be seen to have a close 

 relationship to cells of the grey matter. In this way collateral fibres are given off 

 from the posterior root-fibres and from their longitudinal extensions, some of which 

 penetrate as far as the anterior horn of the same side, others pass through the 

 posterior commissure into the grey matter of the opposite side, others towards the 

 lateral horn and intermediate cell-column, whilst others do not extend beyond the 

 posterior horn, passing towards Clarke's column or the solitary cells of the crescent, 

 while many end in or near the substance of Rolando. These collateral fibres can 

 be seen in sections of the embryonic cord which have been prepared by Golgi's 

 method, to pass into the grey matter in large numbers from the posterior columns, 

 most of the fibres of which are in fact prolongations of posterior roots which 

 have entered the cord. But they are by no means confined to the fibres of the 

 posterior columns, for from all the longitudinal white columns of the cord the 

 same convergence of collateral fibres into the grey matter can be seen (see figs. 16 

 to 20). 



The passage of collateral fibres into the grey matter from the white columns of the cord 

 was first noticed by Golgi, who stated that the fibres of the anterior roots also give off in their 

 passage towards the surface of the cord fine lateral ramuscles which become lost in the adja- 

 cent nervous matter. Other observers have for the most part failed to detect these collaterals 

 of the anterior root-fibres. 



COURSE OF NERVE-FIBRES WITHIN THE SPINAL CORD: CONDUCTING TRACTS. 



It is impossible mechanically to unravel the tracts of nerve-fibres in their passage 

 along the spinal cord, and it is exceedingly difficult to trace the same fibre or fibres 

 for any distance in microscopical sections of the organ. But the task of following 

 out the course of certain sets of fibres has been much facilitated of late years by the 

 application to the subject of certain special developmental and pathological methods 

 of [ observation. Thus it is found that if the development of the spinal cord is 

 carefully observed, the medullary substance of the nerve-fibres is formed later along 

 certain tracts of the white columns than in the rest of the white matter, appearing 

 first in those tracts which are the immediate prolongations of peripheral nerves, and 

 being longest delayed in those which are connected with the higher centres in the 

 brain, so that in transverse sections of the cord these non-medullated tracts are easily 

 distinguishable by their more transparent grey appearance and by their different 

 behaviour with staining fluids (Flechsig). 



The following list (from Kahler) gives the order of formation of the myelin-sheath in 

 the several tracts of the cord : 1. Fibres in anterior column derived from anterior roots. 

 2. Postero-lateral column (its posterior part somewhat later than its anterior part). 3. The 

 lateral column near grey matter (the anterior part first, then the narrow posterior part between 

 pyramidal tract and posterior horn). 4. Postero-mesial column. 5. Direct cerebellar tract. 

 6. Antero-lateral ascending tract. 7. Pyramidal tracts (shortly before birth). 



Another method by which similar results are arrived at consists in tracing the 

 course which the degeneration of the fibres pursues in consequence of a lesion either 

 in the 'encephalon, or in the spinal cord itself, or even in the peripheral nerves ; the 

 lesions being produced by accidental injury, by pathological changes, or experimen- 

 tally in animals. The degenerations which follow are either the result of the 

 Wallerian law that separation of a nerve-fibre from the nerve-cell with which it is 

 connected and from which it has grown out is followed by degenerative changes in 



