SPINAL CHOED OF VERTEBEATA. 



511 



b) Spinal Chord. 

 § 383. 



The spinal chord, which is continuous with the medulla oblongata, 

 is formed from the de- 

 velopment of the lateral 

 halves of the wall of the 

 primitive medullary tube. 

 As the lateral parts in- 

 crease in size an anterior 

 longitudinal fissure is de- 

 veloped. The primitive 

 lumen of the tube is con- 

 verted into the central 

 canal. 



The central appara- 

 tus of the spinal chord 

 occupies the inner parts, 

 and forms a gray medul- 

 lary mass, which is seen, 

 in cross section, to have 

 the form of cornua, which 

 pass forwards and back- 

 wards (Fig. 288, de). 



Owing to the distri- 

 bution of the central 

 apparatus in the central 

 portions of the spinal 



chord, that is, in the gray columns which pass off from the region 

 of the central canal (c), the white medullary mass, which consists 

 of nei've-fibres, is chiefly placed towards the 

 exterior, forming longitudinal columns, which 

 are partly separated from one another by the 

 anterior and posterior longitudinal fissures (ah), 

 and partly by the points at which the roots of the 

 nerves pass out [glii). This arrangement of 

 the white matter is a peculiarity of the spinal 

 chord, and is also a point of difference between 

 it and the ventral chord of the Annulata and 

 Arthropoda, which is of great significance. 



In the Cyclostomata the spinal chord forms a 

 flat band, as it does also in Chimsera ; in most it 

 is more cylindrical in form, and gradually 

 diminishes in size as it passes towards the end 

 of the spinal canal. There are often special 

 enlargements at the points where the larger 

 nerves are given off ; they are very strikingly 



Fig. 288. Transverse section throngh the spinal 

 chord of a Calf, a Anterior, h Posterior longi- 

 tudinal fissure. c Central canal, d Anterior, 

 e Posterior cornua. / Substantia gelatinosa. 

 (J Anterior column of the white substance, li Lateral. 

 i Posterior column, fc Transverse commissures. 



Fig. 289. Braia and 

 spinal chord of Ortha- 

 gosiscus mola (after 

 Arsaky). B Brain and 

 anterior region of the 

 spinal chord of Trig la 

 adriatica (after Tiede- 

 manu) . 



