108 VERTEBRATE ZOOLOGY 



The third and fourth cranial nerves, the oculomotor and the 

 trochlear, are very small and will hardly be found ; they go to 

 muscles of the eyeball. The oculomotor springs from the ventral 

 surface of the midbrain ; the trochlear from the dorsolateral sur- 

 face, between the optic lobes and the cerebellum. 



The fifth, sixth, seventh, and eighth cranial nerves— which are 

 the trigeminal, abducens, facial, and auditory respectively— arise 

 close together from the forward end of the medulla oblongata. 

 The first three of these nerves, together with the anterior end 

 of the sympathetic nerve, are united in the Gasserian ganglion. 

 The trigeminal nerve is the largest of these three ; it arises from 

 the side of the brain just beneath the cerebellum and passes for- 

 ward to the ganglion. The abducens is a very slender nerve which 

 arises from the ventral surface of the medulla near the median line. 

 The facial and auditory nerves arise close together behind the 

 trigeminus. The latter is the larger and passes directly to the 

 auditory capsule ; the former is much smaller and passes alongside 

 the trigeminus to the Gasserian ganglion. 



This ganglion, it will be seen, is not strictly homologous to the 

 same ganglion of higher vertebrates, in which it belongs exclusively 

 to the trigeminal nerve. On this account it is sometimes given 

 another name in the frog and the tailless amphibians and is called 

 the pro otic ganglion. 



Four nerves leave the Gasserian ganglion and at once pass 

 through a large foramen in the skull into the hinder part of the 

 orbit. Pull the eyeball gently forward, cut its muscles and the 

 optic nerve, and remove it. Two of these nerves, the ophthalmic 

 and the maxillo-mandibular, belong to the trigeminal, and two, 

 the palatine and the hyo mandibular, belong to the facial nerve. 

 The abducens becomes a part of the ophthalmic. 



The ophthalmic is a prominent nerve which passes straight for- 

 ward along the upper portion of the orbit to a foramen at its 

 forward end, where it leaves the orbit; it then passes into the 

 nasal capsule and divides into two branches. Near its base this 

 nerve gives off two small branches which represent the abducens 

 nerve and go to the muscles of the eyeball. The maxillo-mandibular 

 passes laterally from the ganglion and almost immediately divides 



