THE STRUCTURE OF THE SPINAL CORD. 543 



the section or injury, and the group of fibres thus degenerating can hardly 

 be considered as forming a tract comparable to the other tracts. . The area 

 probably represents fibres of the posterior root which take a descending course 

 soon after their entrance into the cord. 



II. Ascending tracts, that is to say, tracts in which the degeneration takes 

 place above the section or injury. 



A conspicuous ascending tract of a curved shape (Fig. 123, C.b.) occu- 

 pies the outer dorsal part of the lateral column lying to the outside of the 

 crossed pyramidal tract, between it and the surface of the cord. It appears 

 to begin in the upper lumbar region, being said to be absent from the lower 

 lumbar and sacral cord, and may be traced upward increasing in size through 

 the thoracic and cervical cord to the bulb. In the bulb it may be traced 

 into the restiform body or inferior peduncle of the cerebellum, and so to the 

 cerebellum ; for the restiform body serves, as we shall see, in each lateral 

 half of the brain, as the main connection of the cerebellum with the bulb 

 and spinal cord. Hence this tract is called the cerebellar tract. 



A second important ascending tract occupies the median portion of the 

 posterior columns (Fig. 123, cr., s.lr.), and so far coincides with what we 

 described above as the median posterior column, in the upper regions of the 

 cord, that it may be called the median posterior tract ; it extends along the 

 whole length of the spinal cord, varying at different levels in a manner which 

 we shall presently study, and ending above in the bulb. 



A third ascending tract, called the ascending antero-lateral tract, or tract 

 of Gowers, occupies (Fig. 1 23, asc. a. /.) the outer ventral part of the lateral 

 column. It has somewhat the form of a comma, with the head filling up 

 the angle left between projecting portions of the cerebellar and pyramidal 

 tracts, and the tail stretching away ventrally along the outer margin of the 

 lateral column outside the antero-lateral descending column, the end of the 

 tail often reaching to the anterior roots. It may be traced along the whole 

 length of the cord, but it is not so distinct and compact a tract as the two 

 ascending tracts just mentioned ; the fibres with ascending degeneration, that 

 is to say, the fibres degenerating above the section or seat of injury, are very 

 largely mixed with fibres of a different nature and origin. 



We may further remark that these several tracts differ from each other, 

 in some cases markedly, as to the diameter of their constituent fibres. Thus 

 the cerebellar tract is composed almost exclusively of remarkably coarse 

 fibres. The median posterior tract, on the contrary, is made up of fine fibres 

 of very equable size, while the fibres of the antero-lateral ascending tract 

 are of a size intermediate between the other two. The pyramidal tract, on 

 the other hand, is made up of fibres of almost all sizes mixed together. 



The tracts then which are thus marked out are, as descending tracts, the 

 crossed and the direct pyramidal tracts, with the less distinct or 'important 

 antero-lateral descending tract ; and, as ascending tracts, the cerebellar tract, 

 the median posterior tract, and the less distinct antero-lateral ascending 

 tract. If we suppose all these tracts taken away there is still left a consider- 

 able area of white matter, namely, nearly the whole of the external posterior 

 column, the external anterior column, including the region traversed by the 

 bundles of the anterior roots, and that part of the lateral column which lies 

 between the antero-lateral descending tract and the crossed pyramidal tract 

 on the outside and the gray matter on the inside. From this area of white 

 matter we may put on one side at present the external posterior column, 

 because, as we shall see, this column is largely composed of the fibres of 

 the posterior root which pass through this column, especially through the 

 lateral part of it near the gray matter, on their way to their ultimate desti- 

 nation ; hence the alternative name of posterior root-zone. We may simi- 



