454 



SPINAL NERVES. 



The anterior roots grow rapidly, and soon form elongated 

 cords of spindle-shaped cells with wide attachments to the spinal 

 cord (fig. 267). At first they pass obliquely and nearly hori- 

 zontally outwards, but, before reaching the muscle-plates, they 

 take a bend downwards. 



One feature of some interest with reference to the anterior 

 roots is the fact that they arise not vertically below, 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 commis- 

 sure uniting the dorsal roots, I have not 

 hitherto met with success. It grows thinner 

 and thinner, becoming at the same time 

 composed of fibrous protoplasm with im- 

 bedded nuclei, and finally ceases to be re- 

 cognisable. I can only conclude that it 

 gradually atrophies, and ultimately vanishes. 



After the junction of the posterior and 

 anterior roots the compound nerve extends 

 downwards, 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 dn}. 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 towards colouring reagents, and 

 can be separately traced in the compound nerve. 



So far as has been made out, the development of the spinal 

 nerves of other Vertebrates agrees in the main with that in 

 Elasmobranchii, but no dorsal commissure has yet been discovered, 

 except in the case of the first two or three spinal nerves of the 

 Chick. 



In the Chick (Marshall, No. 353) the posterior roots, during their early 

 stages, closely resemble those in Elasmobranchii, though their relatively 

 smaller size makes them difficult to observe. They at first extend more or 



FIG. 269. TRANSVERSE SEC- 

 TION THROUGH THE DORSAL RE- 

 GION OF A YOUNG TORPEDO EM- 

 BRYO 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. noto- 

 chord; vr. mesoblast cells which 

 will form the vertebral bodies. 



