XERVOUS SYSTEM OF THE VERTEBKATA. 



.373 



.r 



end. They differ from the posterior 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 

 rapidly, and soon form elon- 

 gated cords of spindle-shaped 

 cells with wide attachments 

 to the spinal cord (fig. 207). 

 At first they pass obliquely 

 and nearly horizontally out- 

 wards, but, before reaching 

 the muscle-plates, they take 

 a bend downwards. mp 



One feature of some in- 

 terest with reference to the 



,1 /- ,1 FIG. 268. SECTION THROUGH THE DORSAL 



anterior roots is the tact that EEOIQN op A PRISTIUBUS EMBRY0 . 



they arise not vertically be- ;JJ ._ poaterior root . , p ^ spinal ganglion ; n. 



low, but alternately with the nerve; .r. attachment of ganglion to spinal cord; 



posterior roots: a condition "<- neural canal; mp. muscle-plate; eh. noto- 



i i ... ,i 11, chord; i. investment ot spinal cord, 



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. 205 

 and 200). 



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 



downwards, and may easily be traced for FIG. 26:). TRANSVERSE SEO 



a considerable distance. A special dorsal TION THROUGH^THE^DORSAL^ REGION 

 branch is given off from the ganglion on 



the posterior root (fag. 275 w). According 



to Lowe the fibres of the anterior and pos- SPINAL NERVES. 



AND POSTERIOR ROOTS OF THE 



terior roots can easily be distinguished in 



r. posterior root of spinal 



the higher types by their structure and nerve; ar. anterior root of spinal 

 behaviour towards colouring reagents, and 

 can be separately traced in the compound 



behaviour towards colouring reagents, and uei ; ve .' "'P- mn 



6 notochord; rr. mesoblast 



form the vertebral 



nTve. 



bodies. 



