I 



SPINAL CORD 14s 



derm, the cells of the side walls begin a rapid proliferation, while 

 those of roof and floor plates do not. As a result the sides of the 

 tube extend laterally and especially toward the ventral side so that 

 the floor plate comes to lie at the bottom of a groove, the anterior or 

 ventral fissure (fig. 153, B, v) running the whole length of the cord. 

 At first the roof plate is carried upward by the increase in the sides 

 of the cord, thus increasing the vertical diameter of the central canal. 



Then the dorsal part of the canal closes — the exact steps are 

 uncertain — and later the line of closure is invaded by connective 

 tissue carrying blood-vessels, the result being a marked dorsal sepa- 

 ration between the two halves of the cord, the posterior or dorsal 

 septum (fig. iSA,Fp). 



The cells which line the inside of the cord — floor, roof and sides — 

 retain their epithelial character through life and form the ependjrma 

 which is non-nervous in character. The other cells of the sides be- 

 come differentiated in two directions. Some develop processes 

 which surround and support the other cells, these forming the 

 neuroglia ('glia'). The others form the neuroblasts which are to 

 develop true nervous tissue. 



In the early stages the neuroblasts are without connexions with 

 distant parts and hence cannot function as nerve cells. These con- 

 nexions are established by protoplasmic outgrowths from the neuro- 

 blasts which can extend to enormous distances. These form the 

 afferent and efferent tracts or fibres of each neuron (p. 24). Some 

 of these fibres produced in this way extend out from the cord as 

 nerves (see below), but others run longitudinally for greater or' less 

 distances on the outside of the cord, thus forming an envelope of fibres 

 (the marginal layer) on the outer surface. Since these fibres are 

 medullated (p. 24) and are consequently white, the envelope forms 

 what is known as the white matter of the cord, while the more internal 

 parts, composed largely of the cell bodies, consitute the gray matter. 



In the adult of the higher vertebrates the gray matter, in trans- 

 verse section (fig. 154), is shaped something like the letter H, with 

 the central canal in the middle of the cross bar. The uprights of the 

 H form two pairs of ridges of gray matter which run lengthwise of the 

 cord, known as the posterior and anterior coltmins (formerly cornua) 

 of the cord. Where the cross bar meets the columns there is a third 

 or lateral colimin, differing markedly in relations and function from 

 the others. Since both anterior and posterior columns approach 

 nearer the surface oi the cord than does the rest of the gray matter, 

 zo 



