THE NERVOUS SYSTEM 483 



superior, inferior, and anterior recti and the inferior oblique. Sympathetic 

 non-medullated fibers presumably occur in the oculomotor, but no distinct 

 sympathetic ganglion is formed. 



The trochlearis arises from a motor nucleus in the floor of the meten- 

 cephalon posterior to that of the oculomotor and supplies the superior 

 oblique eye muscle. Its fibers emerge from the medulla near the root of 

 the profundus. The dorsal chiasma of the trochlearis appears to be 

 absent in cyclostomes. 



The trigeminus is so named because, in all vertebrates, it has three 

 chief branches, a sensory ophthalmicus profundus which extends above 

 the eye to the skin on the upper side of the snout, a sensory maxillaris 

 branch to the skin on the side of the snout and the region which cor- 

 responds to the upper jaw of fishes, and a mixed mandibularis branch 

 which supplies the skin and muscles of the first visceral arch. 



Cyclostomes are the only vertebrates in which the profundus branch 

 arises by an independent root. This fact supports the conclusion that 

 the profundus was once an independent segmental nerve and that its union 

 with the trigeminus in all vertebrates above cyclostomes is secondary. 



The motor center of the trigeminus is in the lateral column of the 

 medulla. A unique feature of the trigeminus is a sensory nucleus in 

 the roof of the mesencephalon, the fibers of which bring nerve impulses 

 from mandibular muscles. Most of the sensory fibers of the trigeminus 

 arise from ganglion cells in the large Gasserian ganglion near its root of 

 origin from the medulla. With few exceptions, the sensory nerves of all 

 vertebrates have similar ganglia near their roots of origin. 



The abducens is a somatic motor nerve which emerges from the 

 medulla ventral and posterior to the root of the trigeminus and innervates 

 the posterior or external rectus eye muscle. 



The facialis carries both special and general somatic sensory fibers, 

 and also visceral sensory and motor fibers. The motor fibers arise from 

 an elongated nucleus in the lateral column of the medulla, and supply 

 muscles of the hyoid arch. There are four major branches. The sensory 

 ramus ophthalmicus superficialis innervates the supraorbital series of 

 lateral line organs. The buccalis supplies the infraorbital series. The 

 deep palatine branch is distributed to the skin of the roof of the mouth. 

 The hyomandibular innervates the muscles of the hyoid arch and the skin 

 of the mandibular region. 



The auditory supplies the otic capsule which in cyclostomes is chiefly 

 an organ of equilibration. As might be expected from its origin as a 

 branch of the facial nerve, its roots are closely associated with those 

 of the facial. Since the otic capsule is a modified lateral line organ, the 

 fibers of the auditory nerve belong to the group of lateralis or special 

 somatic sensory components. 



