ASCENDING TRACTS IN THE SPINAL CORD 233 
(a) Descending portion.— Thk is found to occupy an elongated zone in tlie middle ot the 
postero-external column. It is well marked in the middle of the Vth segment, and scattered 
fibres can be traced as far as the lower part of the Vlth segment. Tlnoughout their descending 
course these fibres give off collaterals to the posterior horn. 
(/>) Ascending portion. — Fig. 2, which is at the level of the injured root, shows the entrance 
of degenerated fibres into the postero-external column, and the rich distribution of the reflex 
collaterals of Kolliker, which appear to be chiefly directed towards the postero-lateral group of 
cells. Other evidence tends to show that this group of cells receives special innervation from the 
posterior afferent root fibres. The collaterals are confined to the same side as the root lesion, and 
although more numerous in the IVth segment are found in the segment above, and also, as 
previously stated, in the Vth segment. 
Fig. 2 further shows a marked degeneration in the anterior commissure. The fibres can 
be traced from the anterior horn on the side of the lesion to the white matter between tlie fissure 
and anterior horn of the opposite side. 
This decussating system of degenerated fibres was limited to the region of the injured root. 
In sections higher up it was not possible to distinguish between these fibres and those previously 
noted, which are in relation to the ventral cerebellar tract. Under the name of ' crossed afferent 
tract,' Edinger describes in this position a system of fibres, whose origin is from the posterior 
roots, ascending to terminate in the corpora quadrigemina or optic thalamus. Observers (Mott 
and Langley) who have noted this tract in experimental cases are divided in their opinion as to 
whether the fibres take origin in the posterior roots or in the most dorsal segment of the posterior 
cornu. 
In our case there was no indication that the posterior cornu was injured, and hence this 
observation tends to confirm Edinger's view. 
Sections (Fig. 2) at the site of the injured nerve also showed distinct degeneration in some 
fibres passing from the anterior horn, which were limited to the side of the lesioii. These fibres 
were too numerous to be neglected, but it did not appear that they were continuous with those 
entering the posterior horn. We are therefore unable to give a definite opinion as to their 
precise meaning. 
The ascending portion of the IVth root in the Ilird and higher segments occupies (Fig. 3) 
an elongated area in the median part of the pa^tero-cxternal column, extending ventrally as a 
narrow strip to the base of the posterior horn, while dorsally the area widens and curves outwards 
towards the posterior root zone. In Figs. 4 and 5 the fibres are seen collected around the external 
cuneate nucleus in which they end. 
