IX BLOOD, NERVOUS SYSTEM 327 



slender and is known as the spinal cord while the portion at the anterior 

 end is much enlarged and forms the brain. 



The neural tube originates as a localized thickening of the ectoderm 

 along the dorsal side of the embryo known as the medullary plate. This 

 grows actively and, its central portion being held down by adhesion to 

 the notochord, its edges curve upwards and form the medullary folds, 

 bounding a trough-like neural groove. With continued growth the folds 

 arch in towards one another so that the opening of the groove narrows 

 to a slit and eventually disappears, the edges of the folds undergoing 

 complete fusion. In this way the neural groove is converted into the 

 closed neural tube which soon loses its connexion with the external 

 ectoderm. 



An important point to appreciate is that the surface of the central 

 nervous system which faces inwards towards the central canal is that 

 which originally was part of the external surface of the ectoderm. 



The spinal cord in its fully developed condition is still tubular in 

 form but its cavity, the central canal, is relatively minute while its wall 

 has become enormously thickened and greatly complicated in structure. 

 The inner portion is crowded with ganglion cells (" grey matter ") while 

 the outer part consists for the most part of longitudinally running 

 nerve-fibres (" white matter "). 



From the spinal cord there pass off at intervals — a pair corresponding 

 to each pair of myotomes — large bundles of nerve-fibres known as the 

 spinal nerves. Each of these arises from the spinal cord by two roots, 

 a dorsal and a ventral (Fig. 137, d.r and v.r). The dorsal root shows a 

 distinct swelling on its course, the spinal ganglion {s.g). Each root is 

 composed of nerve-fibres but these show important and characteristic 

 differences in the two roots. A fibre belonging to the ventral root if 

 traced into the spinal cord will be found to originate in a ganglion-cell 

 with branched projections from its body situated towards the ventral 

 side of the spinal cord (Fig. 137, M). In the opposite direction such 

 a nerve -fibre passes outwards along the nerve trunk and eventually 

 terminates in a muscle-fibre {m) : it is clearly a motor or efferent nerve- 

 fibre. 



A nerve-fibre belonging to the dorsal root on the other hand is 

 found to have a spindle-shaped ganglion-cell (Fig. 137, S) — pear-shaped 

 in the higher vertebrates — intercalated on, its course and situated in the 

 spinal ganglion. Peripherally this nerve-fibre is connected not with a 

 muscle-fibre but with a sensory nerve-ending in the skin or elsewhere. 

 It is clearly a sensory or afferent fibre. Traced inwards from the ganglion- 

 cell it is found to enter the spinal cord and to be continued in a T-like 



