NERVOUS SYSTEM OF THE VERTEBRATA. 



373 



Fig. 268. Section through the dobsal 



REGION OF A PkISTIURUS EMBRYO. 



pr. posterior root ; gi).(j. spinal ganglion ; n. 

 nerve ; .t. attachment of ganglion to spinal cord ; 

 nc. neural canal; mp. muscle-plate; ch. noto- 

 chord ; i. investment of spinal cord. 



end. They differ from the postei-ior roots in never shifting their 

 point of attachment to the 

 spinal cord, in not being- 

 united with each other by 

 a commissure, and in never 

 developing a ganglion. 



The anterior roots grow 

 rapidl}^, and soon form elon- 

 gated cords of spindle-shaped 

 cells with wide attachments 

 to the spinal cord (fig. 2G7). 

 At first they pass obliquely 

 and nearly horizontally out- 

 wards, but, before reaching 

 the muscle-plates, they take 

 a bend downwards. 



One feature of some in- 

 terest with reference to the 

 anterior roots is the fact that 

 they arise not vertically be- 

 low, but alternately with the 

 posterior roots : a condition 

 which persists in the adult. 

 They are at first quite separate from the posterior roots ; but about 

 the stage represented in fig. 267 a junction is effected between each 

 posterior root and the corresponding anterior root. The anterior root 

 joins the posterior at some little distance below its ganglion (figs. 265 

 and 266). 



Although I have made some efforts to 

 determine the eventual fate of the com- 

 missure uniting the dorsal roots, I have not 

 hitherto met with success. It grows thinner 

 and thinner, becoming at the same time com- 

 posed of fibrous protoplasm with imbedded 

 nuclei, and finally ceases to be recognisable. 

 I can only conclude that it gradually atro- 

 phies, and ultimately vanishes. 



After the junction of the posterior and 

 anterior roots the compound nerve extends 

 downwrards, and may easily be traced for 

 a considerable distance. A special dorsal 

 branch is given off from the ganglion on 

 the posterior root (fig. 275 cln). According 

 to Lowe the fibres of the anterior and pos- 

 terior roots can easily be distinguished in 

 the higher types by their structure and 

 behaviour towrirds colouring reagents, and 

 can be separately traced in the compound 

 nerve. 



pr 



^ntp 



Fig. 269. Transverse sec- 

 tion through the dorsal region 

 of a young Torpedo embryo to 



SHEW THE origin OF THE ANTERIOR 

 AND POSTERIOR ROOTS OF THE 

 SPINAL NERVES. 



pr. posterior root of spinal 

 nerve; ar, anterior root of spinal 

 nerve; mp. muscle - plate ; ch. 

 notochord; vr. mesoblast cells 

 which will form the vertebral 

 bodies. 



