212 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



curve backwards and upwards to fuse with the optic lobes. The 

 anterior thinner limbs are the optic nerves. They pass through 

 the optic foramina to reach the eyeballs. 



The connection with the optic lobes is a secondary one that is, the optic 

 structures do not originally grow out of them, but fusion occurs later. 



The very small third and fourth pairs belong to the mid-brain. 

 III. The oculo-motor nerves arise, near the middle line, from the 

 front of its floor, and pierce the side-walls of the skull by special 

 foramina to supply most of the eye-muscles. IV. The pathetic 

 (trochlear) nerves are given off from the dorsal surface of the 

 brain just behind the optic lobes. They pass through foramina 

 in the cranial wall, and supply the superior oblique muscles of 

 the eye. 



The remaining six pairs belong to the kind-brain, and, with 

 the exception of the sixth, arise from the sides of the bulb. V. 

 The trigeminal are the largest cranial nerves. Each dilates into 

 a Gasserian ganglion, leaves the skull by the trigeminal foramen, 

 and immediately divides into two (1) The ophthalmic branch, 

 which runs along the inner side of the orbit, and then divides to 

 supply the olfactory capsule and skin of the snout ; and (2) The 

 maxillo-mandibular branch. The latter at once bifurcates into 

 (a) The maxillary nerve, supplying the margin of the upper jaw ; 

 and (&) The mandibular nerve, running round the angle of the 

 mouth and along the outside of the mandible, supplying the skin 

 of that region. Branches from this nerve are also distributed to 

 the muscles of the mouth-floor and to certain muscles which 

 elevate the lower jaw. VI. The very delicate abducent nerves 

 arise close together from the ventral surface of the bulb near its 

 front end. Each comes into close connection with the Gasserian 

 ganglion, separates from it, and leaves the skull by a small aper- 

 ture in front of the trigeminal foramen to supply the external 

 rectus and retractor bulbi muscles of the eye, and also the iris. 

 VII. The facial nerve on each side arises just behind the tri- 

 geminal. and passes out of the skull with it, having previously 

 united closely with the Gasserian ganglion. It divides at once 

 into (1) The palatine nerve, which runs along the floor of the 

 orbit, on its inner side, and supplies the mucous membrane of the 

 mouth-roof ; and (2) The hyomandibular nerve. This runs back 

 round the auditory capsule, crosses the columella, and then 

 descends in the posterior wall of the tympanic cavity to the 



