46 VERTEBRATE ZOOLOGY 



the orbit. The third cranial nerve, the oculomotor, is a small nerve 

 which will be seen arising on the ventral surface of the brain be- 

 neath the optic lobe and passing forward to the orbit, which it 

 enters back of the optic foramen. The fourth nerve, the trochlear, 

 is about the size of the third, and will be seen arising just back of 

 the optic lobe on the side of the brain and passing forward to the 

 orbit, which it enters above the optic foramen. 



The fifth nerve, the trigeminal, and the seventh, the facial, arise 

 together, although without any intermingling of their fibers, from 

 the side of the anterior end of the medulla; this common root 

 will be easily recognized by its large size. The two nerves leave 

 the cranium together. The trigeminal then splits up into the fol- 

 lowing three nerves : ( i ) the ophthalmic nerve, which passes for- 

 ward along the wall of the orbit, dorsal to the optic nerve, to the 

 anterior surface of the head ; (2) the maxillary nerve, which goes 

 to the upper jaw ; and (3) the mandibular nerve, which goes to the 

 lower jaw. The facial nerve passes backward and, dividing into 

 two main branches, supplies the lower jaw, the opercle, and the 

 branchiostegal membrane. 



The sixth nerves, the abducens, are a pair of very small nerves 

 which arise near the midventral line of the brain and pass to 

 the orbits ; they may be seen when the ventral surface of the brain 

 is studied. The eighth nerve, the auditory, arises on the side of 

 the medulla just back of the common root of the trigeminal 

 and facial nerves ; it at once divides into three branches which 

 go to the auditory capsule. The ninth nerve, the glossopharyngeal, 

 and the tenth, the vagus or pneumogastric, arise together on the 

 side of the medulla by three roots. The smallest of these roots is 

 the glossopharyngeal nerve, which passes backward to the first gill 

 arch. The two largest roots join and form the vagus nerve, which, 

 passing posteriorly, supplies the gill arches, except the first, and 

 the heart, stomach, and lateral line. Follow it and its branches 

 as far as possible. 



Exercise 14. Draw the side view of the brain and the cranial nerves, 

 so far as observed, on a scale of 3 or 4. 



Cut the olfactory nerves ; loosen the brain from its attachments, 

 bend it back, and remove it from the body. Study its ventral 



