MORPHOGENESIS OF CENTRAL NERVOUS SYSTEM 821 



gray column. In the region of the central canal the mantle layer forms the 

 dorsal and ventral gray commissures which extend across the nerve cord 

 joining the gray columns in the lateral walls of the cord. Somewhat later, a 

 lateral gray column on either side may be formed between the dorsal and 

 ventral gray columns. 



As the above growth and development of the mantle layer is achieved, the 

 marginal zone of the spinal cord also increases in size as nerve fibers from 

 the developing neurons in the gray columns and in the spinal ganglia of the 

 dorsal roots grow into the marginal layer between the neuroglia elements. 

 Moreover, nerve fibers from developing neuroblasts in the brain grow pos- 

 teriad in the marginal layer of the cord. As the growth and expansion of the 

 dorsal and ventral gray columns toward the periphery of the spinal cord 

 occurs, the marginal layer becomes divided into definite regions or columns 

 known as funiculi. The dorsal funiculus, for example, lies between the dorsal 

 median septum and the dorsal gray column while the ventral funiculus is 

 bounded by the ventral median fissure and the ventral gray column. The 

 lateral funiculus lies laterally between the dorsal and ventral gray columns 

 (fig. 355F). Below the ventral gray commissure, fibers cross from one side 

 of the cord to the other, forming the ventral white commissure. 



Eventually the nerve fibers of each funiculus become segregated into fiber 

 tracts. As a result, the dorsal funiculus becomes subdivided into the two fiber- 

 tract bundles, the fasciculus gracilis near the dorsal medial septum and the 

 fasciculus cuneatus near the dorsal gray column. Other fiber tracts are shown 

 in figure 355F. (Consult Ranson, '39, p. 110.) 



b. Enlargements of the Spinal Cord 



The spinal cord in many tetrapoda tends to show two enlarged areas, viz. 

 (fig. 355G): 



( 1 ) The brachial (cervical) enlargement in the area of origin of the brachial 

 nerves; 



(2) The lumbar (sacral) enlargement in the area of origin of the lumbo- 

 sacral plexus. 



Posteriorly the cord tapers toward a point, and anteriorly, in the region of 

 the first spinal nerve, it swells to become continuous with the myelencephalon. 



c. Enveloping Membranes of the Cord 



Immediately surrounding the spinal cord is a delicate membrane, the pia 

 mater, presumably developed from neural crest cells. More lateral is the 

 arachnoid layer, developed probably from neural crest cells and mesenchyme. 

 Between the pia mater and the arachnoid is the subarachnoid space contain- 

 ing blood vessels, connective tissue fibers, and a lymph-like fluid. Outside 



