NERVOUS SYSTEM 



337 



aquaeductus Sylvii or iter a tertio ad quartum ventriculum, 

 or iter for short. 



The central nervous system gives off paired nerves, and in 

 the trunk and tail these are obviously segmentally arranged, 



Fig. 254. — -The brain of the dogfish, seen from above. 



cb., Cerebellum ; cer., cerebrum ; m.o., medulla oblongata ; olf.l., olfactory lobe ; olf.o., olfactory organ ; 

 op., ophthalmic branches of fifth and seventh nerves; o/>./., optic lobes ; />.s/., pineal stalk ; f./).. restiforn'i 

 body ; sp.c, spinal cord ; sp.n., spinal nerve ; thai., thalamcncephalon ; 3, 4, third and fourth ventricle^ . 

 II.-V., \'II-X., cranial nerves. 



forming the series of spinal nerves characteristic of vertebrates. 

 Each nerve has a dorsal root, consisting of sensorv fibres, whicli 

 goes out of the neural canal b\' a notch in the hind edge of the 

 intercalary piece, and a ventral, of motor fibres, which goes out 

 by a notch in the hind edge of the neural arch. The dorsal root 

 has a ganglion just outside the vertebra, and beyond this the 



