66 COMPARATIVE ANATOMY 



sixth, or abducens nerve, passes to the posterior rectus and gives 

 off a branch to the retractor bulbi. The third nerve rises from 

 the floor of the mid-brain near the median line, the sixth pair 

 from the ventral surface of the medulla, and also close to the 

 median line ; but the fourth pair differs from all the other cranial 

 nerves in arising from the dorsal side of the brain between the 

 medulla and optic lobes. 



The fifth pair of nerves, called the trigeminal, is the largest 

 of the cranial nerves of the frog. The nerve of each side 

 arises from the side of the anterior end of the medulla 

 oblongata, runs outward and forward to the cranial wall, and 

 passes through a foramen which is partly bounded by a notch 

 in the inner side of the pro-otic bone. Just before it reaches 

 the foramen the trigeminal swells out to form the large 

 Gasserian ganglion. After passing through the foramen the 

 nerve runs along the anterior face of the auditory capsule and 

 divides at once into two large branches, the ophthalmic branch 

 and the maxillo-mandibular branch. The ophthalmic branch 

 runs forward close to the cranial wall between it and the 

 eyeball. At the front end of the orbit it divides into two 

 nerves which pass through apertures in the walls of the olfactory 

 capsule and supply the nose and the skin of the front of the 

 head. The maxillo-mandibular nerve, after a short course in 

 front of the auditory capsule, divides into two branches an 

 upper, the maxillary nerve, which runs forward and outward 

 between the eyeball and the lower and outer wall of the 

 orbit to the margin of the upper jaw, and the lower mandi- 

 bular nerve, which turns backward, outward, and downward 

 to the squamosal bone, and passes across to the lower 

 jaw, where it runs forward on the outer side to the 

 symphysis. 



The seventh, or facial nerve, arises from the medulla, close 

 behind the fifth, and runs forward close to the Gasserian 

 ganglion, with which it unites. It leaves the cranium in close 

 company with the mandibular branch of the trigeminal, and 

 divides at once into (i) a palatine branch, which runs forward 

 on the floor of the inner side of the orbit, and eventually 

 makes connections with the maxillary branch of the fifth, and 

 supplies the roof of the mouth ; (2) a hyomandibular branch, 

 which runs outward and backward across the wall of the 

 auditory capsule to reach the columella auris ; crossing over 



