THE STRUCTURE OF THE SPINAL CORD. 559 



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 con- 

 nections in the gray 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 gray 

 matter soon after their entrance into the cord. A representative 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 gray matter of the bulb. 

 Further, each spinal nerve has this representative of its posterior root placed 

 in a definite position in the posterior median column, the arrangement being 

 such, as shown in Fig. 127, 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 arid 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 result following sec- 

 tion of the posterior roots outside the cord ; but it will be understood that 

 similar results follow upon section or of 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 posterior column, reach- 

 ing 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 corre- 

 sponding 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. 



491. The antero-lateral ascending tract (Fig. 127, asc. a. /.) 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 



