894 THE FEATURES OF DIFFERENT REGIONS. [BOOK m. 



the fibres joining the cord in a posterior root, while some, and these 

 we may add are chiefly fine fibres, entering the grey matter 

 directly or passing into the posterior marginal zone, soon make 

 such connections that the degeneration due to the section of the 

 root spreads no farther, a large number, and these chiefly coarse 

 fibres, before they make any such connection pass into and 

 occupy for some length of the cord the external posterior 

 column. We may here remark that though these fibres are 

 spread over the greater part of this column, they do not form 

 the whole of the column; they are mixed up with fibres of a 

 different nature and origin. Of these fibres of the posterior root 

 which thus run in the external posterior column while still 

 dependent for their nutritive activity on the ganglion of the 

 root, some, indeed the greater part, leave the tract and make 

 such connections in the grey matter, that their degeneration 

 ceases; others, forming the smaller part, pass into the median 

 posterior column, and taking up a definite position in that column 

 pursue an unbroken course to the bulb. 



All the fibres therefore of the posterior roots do not end in 

 the grey matter soon after their entrance into the cord. A repre- 

 sentative of each root is carried right up to the bulb by means 

 of the median posterior column ; of the axis-cylinders which leave 

 the ganglion on the root, a certain relatively small number pursue 

 an unbroken course for some little distance through the external 

 posterior column, and for the rest of their way through the 

 median posterior column, along the whole length of the cord above 

 the entrance of the root until they find an ending in the grey 

 matter of the bulb. Further, each spinal nerve has this represent- 

 ative of its posterior root placed in a definite position in the 

 posterior median column, the arrangement being such as shewn 

 in Fig. 104, that the lower (sacral) nerves find their place in the 

 more dorsal and median part of the column, while the nerves 

 above are successively placed in positions more and more ventral 

 and external. 



As far as our knowledge goes at present we are led to believe 

 that this median posterior tract is very largely made up of fibres 

 having this origin. It affords a channel by which afferent impulses 

 are carried straight up the cord from the nerve trunk without 

 making connections on the way. We may repeat that the path is 

 confined to the same side of the cord along its whole length ; 

 there is no crossing over to the other side. 



In the above description we have spoken only of the results 

 following section of the posterior roots outside the cord; but it 

 will be understood that similar results follow upon section of or 

 injury to or disease of the cord itself affecting the posterior 

 columns or the bundles of the roots as they enter the cord. 

 When such a lesion occurs there may be observed in the region 

 of the cord above the lesion a degeneration of the external 



