THE SPINAL COED 323 



the dorsal roof fibers. The cells of the dorsal gray column thus form terminal 

 nuclei for the afferent spinal nerve fibers and they are derivatives of the alar 

 plate of the cord. Dorsal and ventral to the central canal the marginal layer 

 forms the dorsal and ventral gray commissures. In the ventral floor plate nerve 

 fibers cross from both sides of the cord and form the anterior white commissure. 



The Marginal Layer is composed primarily of a framework of neuroglia and 

 ependymal cell processes. Into this framework grow the axis cylinder processes 

 of nerve cells, so that the thickening of this layer is due to the increasing number 

 of nerve fibers contributed to it by ganglion cells and neuroblasts located outside 

 of it. When their myelin develops, these fibers form the white substance of the 

 spinal cord. The fibers have three sources (Fig. 342) : (1) they may arise from 

 the spinal ganglion cells, entering as dorsal root fibers and coursing cranially and 

 caudally in the marginal layer; (2) they may arise from neuroblasts in the ma'ntle 

 layer of the spinal cord (a) as fibers which connect adjacent nuclei of the cord 

 (fasciculi proprii or ground bundles) ; (b) as fibers which extend cranially to the 

 brain; (3) they may arise from neuroblasts of the brain (a) as long descending 

 cerebrospinal tracts from the cortex of the cerebrum; (6) as descending tracts 

 from the brain stem. 



Of these fiber tracts (1) and (2 a) appear during the first month; (2 b) and 

 (3 b) during the third month; (3 a) at the end of the fifth month. 



The dorsal root fibers from the spinal ganglion cells entering the cord dorso- 

 laterally subdivide the white substance of the marginal layer into a dorsal funi- 

 culus and lateral funiculus. The lateral funiculus is marked off by the ventral 

 root fibers from the ventral funiculus (Fig. 309) . The ventral root fibers, as we 

 have seen, take their origin from the neuroblasts of the ventral gray column in the 

 mantle layer. They are thus derivatives of the basal plate. 



The dorsal funiculus is formed chiefly by the dorsal root fibers of the ganglion 

 cells and is subdivided into two distinct bundles, the fasciculus gracilis, median, 

 and the fasciculus cuneatus, lateral in position. The dorsal funiculi are separated 

 only by the dorsal median septum (Fig. 310). 



The lateral and ventral f uniculi are composed of fasciculi proprii or ground 

 bundles, originating in the spinal cord, of ascending tracts from the cord to the 

 brain, and of the descending fiber tracts from the brain. The fibers of these 

 fasciculi intermingle and the fasciculi are thus without sharp boundaries. The floor 

 plate of ependymal cells lags behind in its development, and as it is interposed 

 between the thickening right and left walls of the ventral funiculi, these do not 

 meet and the ventral median fissure is produced (compare Figs. 307 and 310). 



