THE CRANIAL NEBVES 79 



of the brain just below the optic lobe, partially crosses 

 over at the optic chiasma on the under surface of the 

 brain, and then runs outwards to the eyeball. 



The course of the optic nerve has been fully seen in previous 

 dissections. 



3. The motor oculi is a small nerve arising from the ventral 



surface of the brain close to the median line and between 

 the crura cerebri. It supplies four of the muscles 

 moving the eyeball ; viz., the rectus superior, rectus 

 internus, rectus inferior, and obliquus inferior. 



Ovnng to its small size the third nerve is not easily made out 

 in the frog. 



4. The fourth or pathetic nerve is a very slender nerve, 



arising from the dorsal surface of the brain between the 

 optic lobe and the cerebellum, and supplying the obliquus 

 superior muscle of the eyeball. 



The fourth nerve is too small to be dissected satisfactorily in 

 the frog. 



5. The trigeminal is the largest of the cranial nerves in the 



frog. It arises from the side of the anterior part of the 

 medulla, and runs outwards and forwards to the skull 

 wall : just before reaching this it expands into a large 

 swelling the Gasserian ganglion. It then passes 

 through the skull wall immediately in front of the 

 auditory capsule, and divides at once into two main 

 branches. 



i. The ramus ophthalmicus runs forwards through the 

 orbit, lying close to its inner side, between the 

 skull wall and the eye. It passes beneath the 

 rectus superior, but above all the other muscles 

 of the eyeball and the optic nerve. At the 

 anterior end of the orbit it divides into two 

 branches, which pass through the walls of the 

 nasal capsule, and supply the skin of the fore 

 part of the head. 



To trace this branch, dissect from the dorsal surface : cut away 

 carefully with scissors the side wall of the cranium : cut through 

 and turn aside the rectus superior, and find the nerve running 



