THE NERVOUS SYSTEM 45 



ventral side of the diencephalon. Since the retina is an 

 outpushing of the brain wall, the connection it retains with 

 the brain is not a nerve but a tract of the brain. 



c. The third or the oculomotor arises from the floor of 

 the mid-brain and ascends to the orbit near the trochlear 

 nerve. It supplies four of the six muscles of the eye. There 

 is a ciliary ganglion belonging to the sympathetic system 

 which appears as a small brown mass on the oculomotor 

 nerve. 



d. The fourth or trochlear is a small nerve arising 

 dorsally from the midbrain and emerging at the groove 

 between the optic lobes and cerebellum. It supplies the 

 superior oblique muscles of the eye. 



e. The fifth or trigeminus is a very large nerve with 

 four branches. It arises from the medulla oblongata. Its 

 roots are located just behind the auricles of the medulla 

 and are mingled with those of the seventh and eighth 

 nerves. The three branches of the trigeminus are: the 

 maxillary and the mandibular to the muscles of the jaw, 

 and the superficial ophthalmic which runs over the eye 

 to the snout closely united with a similar branch of the 

 seventh. 



f . The sixth or abducens is a slender nerve hidden be- 

 neath the fifth arising near the midventral line. It will 

 be seen as a white ridge on the ventral surface of the ex- 

 ternal rectus muscle of the eye. 



g. The seventh or facial is the nerve of the spiracular 

 cleft. It arises in common with the trigeminus from the 

 anterior end of the medulla and divides into three main 

 branches. Two of these pass to the orbit. It supplies all 

 the ampullae of the head and has numerous branches which 

 go to the lateral line organs. 



h. The eighth or auditory nerve extends from the in- 

 ternal ear to the brain. It enters the anterior end of the 



