874 THE TRACTS OF WHITE MATTER. [BOOK in. 



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 upwards 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 cere- 

 bellum, 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. 100, 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. 100, asc. a. I.) 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 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 considerable area of 

 Avhite 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 descend- 



