THE SPINAL CORD 325 



thus form terminal nuclei for the afferent spinal nerve fibers and they are deriv- 

 atives of the alar plate of the cord. Dorsal and ventral to the central canal the 

 mantle 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 ventral (anterior) 

 white commissure. 



The Marginal Layer is composed primarily of a framework of neurogha 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 extrinsic gangUon cells and neuroblasts. 

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

 cord. The fibers have three sources (Fig. 360) : (1) they may arise from the 

 spinal ganglion cells, entering as dorsal root fibers and coursing craniaUy 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 

 (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 descending tracts 

 from the brain stem, (6) as long descending cerebrospinal tracts from the cortex 

 of the cerebrum. 



Of these fiber tracts (1) 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 funic- 

 ulus and lateral funiculus. The lateral funiculus 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 ganghon 

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

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

 only by the dorsal median septum (Fig. 328). 



The lateral and ventral funicuh 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 

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

 floor plate of ependymal cells lags behind in its development, and, as it is inter- 

 posed between the thickening right and left walls of the ventral funiculi, these do 

 not meet and the ventral median fisstire is produced (cf- Figs. 325 and 328) . 



