296 THE BIOLOGY OF THE FROG chap, 



the frog which arise from the brain and are known conse- 

 quently as cranial nerves. 



The first nerves, counting from before backward, are 

 the olfactory. They arise from the olfactory lobes by two 

 roots, the anterior one emerging from the front end, the 

 posterior running along the ventral side of the lobe nearly 

 to its posterior end. The olfactory nerves pass through 

 small foramina in the ethmoid bone and are distributed to 

 the walls of the nasal chambers. 



The optic^ or second pair of cranial nerves, arise from the 

 thalamencephalon, and, after crossing in the chiasraa, emerge 

 from the skull through foramina in the sides of the chondro- 

 cranium, and are distributed to the eyes. Both the olfactory 

 and the optic nerves are purely sensory. 



The third nerve, oculo-motor, is small and arises from 

 the ventral surface of the crura cerebri. It emerges from the 

 skull through a small foramen near the opening from the 

 optic nerve, and innervates four of the muscles of the eye- 

 ball, the rectus superior, rectus inferior, rectus medialis, and 

 obliquus inferior. After giving off a branch to the rectus 

 superior, the third nerve becomes connected with the ciliary 

 ganglion ; this ganglion also receives a branch from the 

 ophthalmic division of the fifth nerve, and gives off the small 

 ciliary nerves to the retractor bulbi muscle and the tunics 

 of the eye. 



The fourth nerve, the trochlearis, is very small ; it arises 

 from the dorsal side of the brain, between the optic lobes 

 and the cerebellum. It leaves the skull through a special 

 foramen a little above the optic nerve and is distributed to 

 the superior obhque muscle of the eye. Both the third and 

 the fourth nerves are exclusively motor. 



The fifth nerve, the trigeminus or trifacial, is one of the 

 largest of the cranial nerves. It arises from the sides of the 



