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DEVELOPMENT OF THE NERVOUS SYSTEM 735 



tapering part of the cell undergoes still further elongation, and forms the axis- 

 cylinder of the cell. 



The lateral walls of the medulla spinalis continue to increase in thickness, and 

 the canal widens out near its dorsal extremity, and assumes a somewhat lozenge- 

 shaped appearance. . The widest part of the canal serves to subdivide the lateral 

 wall of the neural tube into a dorsal or alar, and a ventral or basal lamina (Figs. 642, 

 643) , a subdivision which extends forward into the brain. At a later stage the ventral 

 part of the canal widens out, \vhile the dorsal part is first reduced to a mere slit 

 and then becomes obliterated by the approximation and fusion of its walls; the 

 ventral part of the canal persists and forms the central canal of the adult medulla 

 spinalis. The caudal end of the canal exhibits a conical expansion which is know r n 

 as the terminal ventricle. 



The ventral part of the mantle layer becomes thickened, and on cross-section 

 ppears as a triangular patch between the marginal and ependymal layers. This 

 thickening is the rudiment of the anterior column of gray substance, and contains 

 many neuroblasts, the axis-cylinders of which pass out through the marginal layer 

 and form the anterior roots of the spinal nerves (Figs. 640,642, 643) . The thickening 

 of the mantle layer gradually extends in a dorsal direction, and forms the posterior 

 column of gray substance. The axons of many of the neuroblasts in the alar lamina 

 run forward, and cross in the floor-plate to the opposite side of the medulla spinalis; 

 these form the rudiment of the anterior white commissure. 



About the end of the fourth week nerve fibers begin to appear in the marginal 

 ayer. The first to develop are the short intersegmental fibers from the neuro- 

 blasts in the mantle zone, and the fibers of the dorsal nerve roots which grow into 

 the medulla spinalis from the cells of the spinal ganglia. By the sixth week these 

 dorsal root fibers form a well-defined oval bundle in the peripheral part of the alar 

 lamina; this bundle gradually increases in size, and spreading toward the middle 

 line forms the rudiment of the posterior funiculus. The long intersegmental fibers 

 begin to appear about the third month and the cerebrospinal fibers about the fifth 

 month. All nerve fibers are at first destitute of medullary sheaths. Different 

 groups of fibers receive their sheaths at different times the dorsal and ventral 

 nerve roots about the fifth month, the cerebrospinal fibers after the ninth month. 



By the growth of the anterior columns of gray substance, and by the increase 

 in size of the anterior funiculi, a furrow is formed between the lateral halves of the 

 cord anteriorly; this gradually deepens to form the anterior median fissure. The 

 mode of formation of the posterior septum is somewhat uncertain. Many believe 



at it is produced by the growing together of the walls of the posterior part of the 

 central canal and by the development from its ependymal cells of a septum of 

 fibrillated tissue w r hich separates the future funiculi graciles. 



Up to the third month of fetal life the medulla spinalis occupies the entire 

 length of the vertebral canal, and the spinal nerves pass outward at right angles 

 to the medulla spinalis. From this time onward, the vertebral column grows more 

 rapidly than the medulla spinalis, and the latter, being fixed above through its 

 continuity with the brain, gradually assumes a higher position within the canal. 

 By the sixth month its lower end reaches only as far as the upper end of the sacrum; 

 at birth it is on a level with the third lumbar vertebra, and in the adult with the 

 lower border of the first or upper border of the second lumbar vertebra. A delicate 

 filament, the filum terminale, extends from its lower end as far as the coccyx. 



The Spinal Nerves. Each spinal nerve is attached to the medulla spinalis by 

 an anterior or ventral and a posterior or dorsal root. 



The fibers of the anterior roots are formed by the axons of the neuroblasts 

 which lie in the ventral part of the mantle layer; these axons grow out through the 

 overlying marginal layer and become grouped to form the anterior nerve root 

 (Fig. 641). 



