288 HAND-BOOK OF PHYSIOLOGY. 



from that of the nerves which arise from them. These nerve-centres are 

 developed entirely from the epiblast (possibly, however, a portion of the 

 spinal cord originates in the mesoblast); while the nerves, as we have 

 seen, are formed from mesoblast. The spinal cord is developed out of 

 the primitive medullary tube which results from the folding in of the 

 dorsal laminae (m, Fig. 411). 



Soon after it has closed in, this tube is found to be somewhat oval in 

 section, with a central canal, which, in sections, presents the appearance 

 of an elongated slit, slightly expanded at each end. The two opposite 

 sides unite (Fig. 445) in the centre of the slit, dividing it into an anterior 

 portion (the permanent central canal of the cord) and a posterior, which 

 makes its way to the free surface, and persists as the posterior fissure of 

 the cord, lodging a very fine process of pia mater. 



At this period the cord consists almost entirely of grey matter, but the 

 white matter, which is derived probably from the surrounding mesoblast, 

 becomes deposited around it on all sides, growing up especially on the 



FIG. 445. Diagram of development of spinal cord; cc, central canal; a/, anterior fissure; pf, pos- 

 terior fissure; <?, grey matter; ty, white matter. 'For further explanation see text. 



anterior surjace of the cord into the two anterior columns. These are 

 separated by a fissure (anterior fissure of cord), which of course deepens 

 as the columns bounding it become more prominent (Fig. 445). 



By the development of various commissures, the cord is completed. 



When it first appears, the spinal cord occupies the whole length of the 

 medullary canal, but as development proceeds, the spinal column grows 

 more rapidly than the contained cord, so that the latter appears as if 

 drawn up till, at birth, it is opposite the third lumbar vertebra, and in 

 the adult opposite the first lumbar. In the same way the increasing 

 obliquity of the spinal nerves in the neural canal, as we approach the 

 lumbar region, and the ' 'cauda equina" at the lower end of the cord, are 

 accounted for. 



Brain. We have seen (p. 257, Vol. II.) that the front portion of the 

 medullary canal is almost from the first widened out and divided into 

 three vesicles. From the anterior vesicle (thalamencephalon) the two 

 primary optic vesicles are budded off laterally: their further history will 

 be traced in the next section. Somewhat later, from the same vesicle 

 the rudiments of the hemispheres appear in the form of two outgrowths 

 at a higher level, which grow upward and backward. These form the 

 prosencephalon. 



