CH. LIX.] 



THE NERVOUS SYSTEM 



859 



S N 



rest of it the spinal cord ; the canal itself is seen in the adult as the 

 ventricles of the brain and central canal of the spinal cord. The 

 nerves are formed of epiblast too ; they are outgrowths from masses 

 of cells called neuroblasts, the primitive nerve-cells. In the case, 

 however, of the olfactory and optic nerves we have not to deal with 

 solid outgrowths, but with hollow protrusions from the brain, which 

 become solid at a later stage. 



The Spinal Cord. The cavity formed by the closure of the 

 neural canal soon becomes a cleft running from before backwards. It 

 is bounded at first by columnar epithelium ; these cells afterwards 

 become ciliated ; on their exterior is a homogeneous basement mem- 

 brane. The wall soon becomes thicker, and the basement membrane 

 is thus separated further and further from the central canal. This 



increase in thickness is due in part to the 

 increase in length of the columnar cells: 

 in part to the appearance" of new cells. 

 The inner part of the columnar lining 

 retains its palisade-like character, and 

 forms ultimately the lining epithelium of 

 the central canal. The cells are called 

 spongioblasts. The external ends of the cells 

 break up into a reticulum called the my- 

 elospongium, and this is limited externally 

 by the basement membrane at the circum- 

 ference. The myelospongium forms the 

 neuroglia. 



Between the inner ends of the spongio- 

 blasts (fig. 656, S) numerous rounded cells 

 called germinal cells (G) next appear. These 

 rapidly divide, and so form neuroblasts (N). The neuroblasts are 

 pear-shaped; each has a large oval nucleus, and its tapering stalk 

 is directed towards the outer surface of the 

 cord; the process ultimately pierces the 

 basement membrane (fig. 657). These are 

 the primitive nerve-cells ; their processes are 

 the axis cylinder processes which grow out 

 as nerve-fibres. The nerve sheaths are formed 

 later (see pp. 692-697). 



The neuroblasts collect into groups, one of 

 which, especially large, is at the situation of 

 the future anterior horn ; the processes of the 

 primitive nerve-cells pass out of the cord as 

 the beginnings of the anterior roots (fig. 658). The somewhat 

 oblique coursing of these fibres before they leave the cord forms 

 the beginning of the anterior white column. The posterior white 



FIG. 656. Inner ends of spongio- 

 blasts (S), with germinal cells 

 (G) between them. NN, neuro- 

 blasts which have resulted from 

 the division of a germinal cell ; 

 M, myelospongium formed by 

 the branching outer ends of the 

 spongioblasts. (After His.) 



FIG. 657. Three neuroblasts, 

 each with a nerve-fibre pro- 

 cess, growing out beyond the 

 basement membrane of the 

 embryonic spinal cord. (After 

 His.) 



