THE NERVOUS SYSTEM 353 



The so-called nervus terminalis appears not to be an independent nerve, 

 but a component of the olfactory. The remaining neurites of the olfactory 

 are processes of neurosensory cells in the olfactory epithelium. 



The optic nerve develops in correlation with the eye, the retina, from 

 which the optic nerve fibers arise, being a segregated part of the wall 

 of the telencephalon. Some of its fibers cross below the brain in front of 

 the infundibulum to form the optic chiasma. The optic nerves, after 

 entering the wall of the diencephalon, pass by way of the optic thalami 

 to their reflex centers in the roof of the mesencephalon. 



The oculomotor, a somatic motor nerve with its nucleus or motor 

 center in the base of the mid-brain, innervates four eye muscles, the 

 superior, inferior, and anterior recti and the inferior obUque. 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 the nerve's 

 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 



