THE CENTRAL NERVOUS SYSTEM. 



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

 these parts are to be regarded as the walls of the primary neural tube, modified by 

 unequal growth and expansion, which even after acquiring their definite relations 

 enclose the remains of the canal, as represented by the system of ventricular spaces. 

 In contrast to the spinal segment of the neural tube, which always remains a rela- 

 tively simple cylinder, the spinal cord, the cephalic segment early differentiates into 

 three primary cerebral vesicles, the anterior and posterior of which subdivide, so that 

 five secondary brain-vesicles are present. Coincidently marked flexure of the 

 cephalic segment occurs at certain points and in consequence this part of the neural 

 tube becomes bent upon itself to such a degree that the axis of the anterior vesicle 

 lies almost parallel with that of the spinal segment (Fig. 912). From the five 

 secondary divisions of the flexed and sinuously bent cephalic segment of the neural 

 tube are developed the fundamental parts of the brain in the manner presently to be 

 described (page 1060), whilst from the relatively straight spinal segment proceeds the 

 development of the spinal cord, in 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 (medulla spinalis) is that part of the central nervous system, or 

 cerebro-spinal axis, which lies within the vertebral canal. Its upper limit, where it 

 becomes continuous with the medulla oblongata, is in a measure conventional, since 

 there is no demarcation on the cord itself to indicate exactly its junction with the brain. 

 Accurately considered, the superior limit of the cord may be assumed to correspond 

 with the emergence of the uppermost root-fibres of the first spinal nerve which pass 

 out between the atlas and the skull ; this level also corresponds to the lowest strands 

 of the pyramidal decussation of the medulla oblongata and to the upper border of the 

 posterior arch of the atlas. For practical purposes, however, the lower margin of 

 the foramen magnum defines with sufficient accuracy the upper limit of the spinal 

 cord. Below, the spinal cord terminates somewhat abruptly in a pointed end, the 

 conus medullaris, that usually ends opposite the disc between the first and second 

 lumbar vertebrae. The level to which the cord extends inferiorly, however, is subject 

 to considerable variation, very rarely being as high as the middle of the body of the 

 last thoracic vertebra (Moorhead), or as low as the upper border of the body of the 

 third lumbar vertebra (Waring). In the female subject the spinal cord, although 

 absolutely shorter than in the male, extends to a relatively lower level in the vertebral 

 canal. Marked bending of the spine produces slight alterations in the position of the 

 cord, during strong flexion an appreciable ascent of the lower end taking place. The 

 relation of the cord to the vertebral canal varies at different periods. Until the third 

 month of foetal life the cord occupies the entire length of the canal, but subsequently, 

 owing to the more rapid lengthening of the spine than of the spinal cord, the latter no 

 longer reaches to the lower limit of the canal and, therefore, apparently rises, so that 

 by the sixth foetal month the lower end of the cord lies opposite the first sacral vertebra, 

 and at birth terminates usually on a level with the body of the third lumbar vertebra. 



Measured from its upper conventional limit to the lower end of the conus medullaris, the 

 spinal cord in the adult male has an average length of 45 cm. (17^ in.), and in the female of 

 43.7 cm. (i7>4^ in.), in both sexes the proportion of the length of the cord to that of the pre- 

 sacral spine being approximately as 64 : 100 (Ziehen). The cord-length bears no constant rela- 

 tion to stature, although in a general way tall individuals may possess long cords. The weight 

 of the spinal cord, stripped of its membranes and nerves, is something less than 30 grammes 

 (i oz.), or about 1-2000 of the body-weight. Its proportion to the weight of the brain is i :43. 

 When fresh the spinal cord possesses a soft cheesy consistence and a specific gravity of 1035. 



