THE CEEEBRAL NERVES. 71 



the chiasma, behind which lies the pituitary body. III. Oculomn- 

 wrins. lY . Pathetkvs. V. The trigeminal, with which the aM?(ce»'s 

 (VI.), facialis (VII.), and the upper end of the sympathetic ( VS.), 

 are closely connected Branches of this nervous plexus are T'.a, 

 ihe nasal and ophthalmic branches of tlie fifth and the ahdncens. 

 r, h, c, d, the palatine, maxillary, and mandibular branches of the 

 fifth. V, e, the tympanic branch into which tlie proper facial nerve 

 ( VII.J enters, and, with a branch of the vagus, forms the so-called 

 facial nerve of the Frog, F. VIII. The auditory nerve. A' , with 

 its branches A'l, A'-', A^, A*, represents the glossopharyngeal and 

 the vagus. The medulla oblongata {Myekncephulon) ends, and the 

 medulla spinalis {Myelon) begins, about the region marked by the 

 letter 31. M 1-U), the spinal nerves. 3/ 2, the brachial nerves, 

 M 7, S, 9, the ischiatic ple.xus, from which proceed the crural 

 (N. c.)and ischiatic (N.z.) nerves. S. The trunk ot the sympa- 

 thetic. S.M. The communicating branches with the spinal ganglia. 

 S 1-10. The sympathetic ganglia. 



the vertebrae tlirougli, or between, which they pass ottt 

 (Fig. 23). Each nerve has two roots, one from the dorsal, and 

 one from the ventral, region of its half of the cortj. The 

 former root has a ganglionic enlargement, and only con- 

 tains sensory fibres ; the latter has no ganglion, and ex- 

 clusively contains motor fibres.* After leaving the vertebral 

 canal, each spinal nerve usually divides into a dorsal and a 

 ventral branch ; but, in the Ganoid fishes, each of these 

 branches is a distinct nerve, arising by its own proper roots. 



The Cerebral Nerves.— The greatest number of pairs of 

 nerves ever given off from the vertebrate brain is twelve, in- 

 cluding the so-called olfactory nerves, and the optic nerves, 

 which, as has been seen, are more properly diverticttla of 

 the brain, than nei-ves in the proper sense of the word. 



The olfactory "nerves" (olfadoriij constitute the first 

 pair of cerebral neiwes. They always retain their primary 

 connection with the cerebral hemispheres, and frequently 

 contain, throughout life, a cavity, the olfactory ventricle, 

 which communicates with the lateral ventricle. 



The optic " nerves " (opticij are the second pair of cere- 

 bral nerves. In the Lampreys and Hags fMarsipohranchii) 

 these nerves retain their embryonic origin from the thala- 



* Amphiuxus appears to be an exception to this, as to most other, 

 rules of Vertebrate anatomy. 



