THE SPINAL CORD 325 



plate, nerve fibers cross from both sides of the cord and form the ventral 

 (anterior) white commissure. 



The Marginal Layer is composed primarily of a framework of neuro- 

 glia and ependymal-cell, processes. Into this framework grow the axons 

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

 number of nerve fibers contributed to it by extrinsic ganglion cells and 

 neuroblasts. When their myelin develops, these fibers form the white 

 substance of the spinal cord. The fibers have three sources (Fig. 360) : 

 (i) 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 mantle layer of the spinal cord, (a) as 

 fibers which connect adjacent nuclei of the cord (fasiculi proprii or ground 

 bundles), or (b) as fibers which extend upward to the brain; (3) they may 

 arise from neuroblasts of the brain, (a) as descending tracts from the 

 brain stem, or (b) as long, descending cerebrospinal tracts from the cortex 

 of the cerebrum. 



Of these fiber tracts, (i) and (2 a) appear during the first month; 

 (2 b) and (3 a) during the third month; (3 b) 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 funiculus and lateral funiculus. The lateral f uniculus is marked off 

 by the ventral root fibers from the ventral funiculus (Fig. 327). 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 fasiculus 

 gracilis, median in position, and the fasciculus cuneatus, lateral.. The 

 dorsal funiculi are separated only by the dorsal median septum (Fig. 328). 



The lateral and ventral funiculi are composed: (i) of fasciculi proprii, 

 or ground bundles, originating in the spinal cord; (2) of ascending tracts 

 from the cord to the brain; (3) 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 (cf. Figs. 325 and 328). 



The development of myelin in the nerve fibers of the cord begins late in the fourth 

 month of fetal life and is completed between the fifteenth and twentieth years (Flechsig, 

 Bechterew) . Myelin appears first in the root fibers of the spinal nerves and in those of the 

 ventral commissure, next in the ground bundles and dorsal funiculi. The cerebrospinal 

 (pyramidal) fasciculi are the last in which myelin is developed; they are myelinated during 



