424 THE ORGANS 



in the adult and are known as ependymal cells; others move away 

 from the central canal and become neuroglia cells. 



Still other of the cells of the neural tube are destined to become 

 neurones, and as such are known as neuroblasts. From the neuro- 

 blast a neurofibrillated process grows out — the future axone. Den- 

 drites which at this stage are absent develop later in a similar manner, 

 i.e., by extensions of the cell protoplasm.^ The neuroblasts soon 

 leave their original position near the lumen and pass outward along 

 the spaces between the elongated ependymal cells, but their bodies 

 do not usually penetrate the marginal veil. Some may, however, 

 pass through and even leave the neural tube along with the efferent 

 roots. The axones of many neuroblasts located in the ventral part 

 of the neural tube pierce the marginal veil and leave the neural tube 

 as the efferent root fibres. These nerve cells together with many of 

 the sympathetic neurones (see below) are the efferent peripheral neu- 

 rones. Their axones pass to sympathetic ganglia or to various struc- 

 tures (muscles, glandular epithelia) the activities of which they affect. 

 Such structures may be collectively termed effectors. The axones of 

 other neuroblasts do not completely pierce the marginal veil, but are 

 directed upward and downward within it, thus forming fibres con- 

 necting various parts of the central nervous system. Axones of still 

 other neuroblasts, especially in suprasegmental structures (see p. 

 426) , are directed toward the lumen. All such neurones whose axones 

 do not leave the neural tube may be termed intermediate or central 

 neurones, as distinguished from the peripheral neurones: inter- 

 mediate, because they serve as intermediate links connecting, within 

 the central nervous system, the terminations of the afferent per- 

 ipheral neurones (see below) with the bodies of the efferent peripheral 

 neurones; central, because they are confined to the central nervous 

 system. Later becoming medullated, their axones constitute the 

 great majority of the fibres of the white matter of the central nervous 

 system. 



During the closure of the neural groove, groups of cells from the 

 crest of each neural fold become separated from the rest of the de- 

 veloping nervous system. Some of these cells form the cerebrospinal 

 ganglia, while, according to most authorities, others migrate still 

 further from the neural tube and form the various sympathetic ganglia. 

 Some cells (neuroblasts) develop into the nerve cells of the cerebro- 



axone 



^According to other views, other cells may participate in the formation of the 

 ne, and the dendrites may anastomose with other neuroblasts (see Chap. VI). 



