THE CENTRAL NERVOUS SYSTEM. 



THE central nervous system includes the spinal cord and the brain. 

 In principle these are the walls of the primary neural tube, modified by 

 unequal growth and expansion, which even after acquiring definite relations 

 enclose the remains of the tube, as represented by the brain-ventricles and 

 the central canal of the cord. In contrast to the spinal segment of the 

 neural tube, which always remains a relatively simple cylinder, the cephalic 

 segment early differentiates into the cerebral vesicles, marked flexure occur- 

 ring coincidently at certain points. From the sinuously bent cephalic seg- 

 ment are developed the fundamental parts of the brain, while from the 

 relatively straight spinal segment proceeds the development of the spinal 

 cord, during which process growth and differentiation convert the originally 

 thin-walled tube into an almost solid cylinder, the minute central canal alone 

 remaining as the representative of the once conspicuous lumen. 



THE SPINAL CORD. 



The spinal cord, or medulla spinalis, is that part of the central nervous 

 system, or cerebro-spinal axis, which lies within the vertebral canal. After 

 removal of its protecting membranes and the attached root-fibres of the 



Posterior median septum 



Posterior column 



Posterior root-furrow 



. Posterior 

 root-fibres 



Caput cornu 



Cervix cornu 



Lateral corni 



Basis cornu' 



Caput cornu 



1 



Lateral column 



Central canal 

 jjjp in gray 



commissure 



Anterior median fissure Anterior column 



Anterior white commissure 



FIG. 313. Cross-section of child's cord through thoracic region, showing arrangement of gray and white 

 matter and subdivision of the latter into columns. X 13. 



spinal nerves, the spinal cord is seen to be a flattened cylinder, so that the 

 antero-posterior diameter is always less than the transverse one; its outline 

 in cross-sections, therefore, is not circular but more or less oval. Its width, 

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