506 



NERVOUS SYSTEM. 



Fig. 259. Longitudinal Section of the 

 white and Kniy substance of the 

 spinal cord, through the middle of the 

 lambar enlargement. Mag. 14 diain. 



Within and along the outer border of the cervix are several thick bundles of longitudinal 

 fibres, represented in the adjoining figure by the dark spots ; other bundles of the same kind 

 may be seen in the gray substance along the line of junction of the caput with the cervix cornu 

 (fig. 258). 



The anterior horn of the gray substance in the cervical and lumbar swellings, where it gives 

 origin to the nerves of the extremities, is much larger than in any other region, and contains 

 several distinct groups of large and variously shaped cells. This is well shown on comparing 

 the above figures. 



The tradus mtermedio lateralis (fig. 257) extends 

 from the upper part of the lumbar to the lower part 

 of the cervical enlargement, and consists of variously 

 shaped cells, which are smaller than those of the 

 anterior cornu. In the neck above the cervical en- 

 largement, a similar tract reappears, and is traversed 

 by the lower part of the spinal accessory nerve. 



Origin of the Spinal Nerves. The posterior roots 

 are larger than the anterior; but their component 

 filaments are finer and more delicate. They are all 

 attached immediately to the posterior columns only, 

 and decussate with each other in all directions through 

 the columns ; but some of them pass through the 

 gray substance into both the lateral and anterior 

 columns. Within the gray substance, they run lon- 

 gitudinally upwards and downwards ; transversely 

 through the posterior commissure to the opposite 

 side ; and into the anterior cornu of their own side 

 (figs. 258 and 259). 



The anterior roots are attached exclusively to the 

 anterior columns, or rather to the anterior part of ihe 

 antero-lateral columns ; for there is no antero-lateral 

 fissure dividing the anterior from the lateral column. 

 Within the gray substance, the fibrils cross each other, 

 and diverge in all directions like the expanded hairs 

 of abrush (figs. 258 and 259), some of them running more 

 or less longitudinally upwards and downwards ; and 

 others decussating with those of the opposite side 

 through the anterior commissure in front of the cen- 

 tral canal. 



All the fibres of both roots of the nerves proceed 

 through the white columns into the gray substance, 

 with, perhaps, the exception of some which appear to 

 run longitudinally in the posterior columns ; but 

 whether these latter fibres of the posterior roots ulti- 

 mately enter the gray substance of the cord after a 

 very oblique course, or whether they proceed upwards 

 to the brain, is uncertain. 1 / 



Ant?' 



Cpliumi> 



THE CENTRAL CANAL. 



In the foetus, until after the sixth month, a canal, continuous with the general 

 ventricular cavity of the brain, extends throughout the entire length of the spinal 

 cord, formed by the closing-in of a previously open groove. 



In the adult, this canal can only be seen at the upper part of the cord, extending 

 from the point of the calamus scriptorius, in the floor of the fourth ventricle, for 

 about half an inch down the centre of the cord, where it terminates in a cul-de- 

 sac; the remnant of the canal being just visible in a section of the cord, as a 

 small, pale spot, corresponding to the centre of the gray commissure, its cavity 

 being lined with a layer of cylindrical ciliated epithelium. In some cases, this 

 canal remains pervious throughout the whole length of the cord. 



1 The above description, and accompanying illustrations, kindly furnished me by my friend, 

 Mr. Lockhart Clarke, embodies a condensed account of his laborious and extensive observations 

 on the structure of the spinal cord. For further information on this subject, vide Phil. Trans. 

 ,18511853, Part iii. ; 1858, Part i. ; 1859, Part i. 



