are said to have twelve, each assigned a roman numeral 

 (Figure 13-3). 



The terminal nerve (0) was described, after the other 

 cranial nerves had been numbered, by F. Pinkus in 1895 

 from the dipnoan, Protopterus. This nerve is closely associated 

 with the olfactory nerve and should not be confused with 

 the vomeronasal division of that nerve. The fibers are dis- 

 tributed to the nasal septum and the area around the ex- 

 ternal naris. The nervus terminalis is presumed to be a 

 sensory nerve (general cutaneous), with a ganglion near or 

 at its base, but it may be a part of the autonomic system 

 (Figure 13-3). This nerve is not apparent in the mammal. 

 The olfactory nerve (I) arises from the sensory cells of 

 the olfactory epithelium in the olfactory chamber or cap- 

 sule. Its vomeronasal division arises from the separate area 

 of sensory cells of Jacobson's organ. The olfactory nerve 

 enters the olfactory bulb, the anterior part of the telence- 

 phalon, where its fibers synapse with those of the cells form- 

 ing the olfactory tract which leads back to the olfactory 

 lobe (nucleus) and other areas of the brain. 



The optic nerve (II) is a brain tract rather than a nerve 

 since the retina of the eye is a part of an evagination of the 

 wall of the diencephalon (see below under eye). The fibers of 

 the optic nerve arise from a ganglionic layer in the surface 

 of the retina — there are two other layers of neurons between 

 these ganglion cells and the sensory cells. The fibers pass 

 over the inner surface of the retina to the blind spot where 

 they form the optic nerve which passes out through the wall 

 of the eyeball to the brain. At the optic chiasma these fibers 

 decussate (cross to the opposite side of the brain) or not, ac- 

 cording to their area of origin, and pass up through the 

 optic tracts to the lateral geniculate nuclei of the thalamus. 

 A few of the fibers continue on to the superior colliculi. 

 Synapse in the lateral geniculate nuclei switches the optic 

 sensory area from the mesencephalon to a part of the cere- 

 bral cortex. 



In mammals the fibers from the medial half of the retina 

 of each eye cross over, or decussate, and accompany the 

 fibers from the lateral half of the retina of the opposite eye 

 into the brain. Decussation of the optic nerve fibers is com- 

 plete in lower forms — all fibers pass to the opposite side. As 

 a generality one can say that the left side of the brain serves 

 the right side of the body. 



The oculomotor nerve (III) supplies the superior, infe- 

 rior, and internal rectus muscles of the eyeball. This motor 

 nerve arises from a small nucleus in the mesencephalic 

 brain stem. This nerve also contains proprioceptive fibers 

 from the eye muscles to the central nervous system. These 

 sensory fibers are employed in the tonal (tension) reflexes of 

 these muscles. There are also preganglionic fibers emerging 

 from the eyeball to join the oculomotor nerve. These fibers 

 lead to the ciliary ganglion and are involved in the opera- 

 tion of the smooth muscles of the ciliary body of the lens 

 and the iris. 



The trochlear nerve (IV) arises in the posterior part of 

 the floor of the mesencephalon near the ventral commissure. 



The fibers extend dorsally and somewhat posteriorly from 

 their nucleus, within the walls of the mesencephalon until 

 they reach the mid-dorsal line. Here they emerge and de- 

 cussate. Each now passes down and forward to the eye re- 

 gion. This nerve innervates the superior oblique muscle, and 

 contains somatic sensory fibers involved in the propriocep- 

 tive reflexes of that muscle. 



The trigeminal nerve (V) is a large complex unit involv- 

 ing sensory and motor fibers. It derives its name from its 

 three branches. The ophthalmicus or profundus branch 

 (Vi) arises from the Gasserian (or semilunar) ganglion; it 

 extends through the orbit to the snout region. The maxil- 

 lary branch (Vo) arises from the same ganglion and is com- 

 posed of sensory elements like the profundus. It passes down 

 beneath the orbit and forward into the maxilla. The mandi- 

 bular branch ( V3) extends down from the Gasserian ganglion 

 behind the orbit and into the mandible through the mandi- 

 bular foramen. 



Primarily the trigeminal is a sensory nerve but it has a 

 visceral motor component. The motor portion arises from a 

 nucleus in the floor of the metencephalon and extends out 

 to the jaw muscles. The interrelationships between motor 

 and sensory components in the mandibular branch are sec- 

 ondary ones. The general visceral fibers (proprioceptive) 

 of this nerve pass to a nucleus in the roof of the mesenceph- 

 alon. A sensory nucleus within the brain is unique but 

 not uncommon in invertebrates and Amphioxus. 



The abducens nerve (VI) arises in the caudoventral part 

 of the pons. The nerve passes out then forward from its 

 nucleus to innervate the lateral rectus muscle of the eyeball. 

 This nerve, although primarily motor, contains some pro- 

 prioceptive sensory fibers. 



The facial nerve (VII) is primarily motor with some 

 sensory fibers having their cell bodies in the geniculate 

 ganglion. This nerve innervates the muscles derived from 

 the hyoid arch, and the dermal muscles of the face and 

 scalp. The special visceral sensory fibers are from the 

 taste buds of the anterior two-thirds of the tongue; these 

 pass through the chorda tympani division of the nerve. 

 General visceral sensory fibers from the submaxillary and 

 sublingual salivary glands pass through the hyoid division 

 of this nerve to reach the brain. Motor fibers of the facial 

 nerve also activate the salivary glands. 



The auditory, or acoustic, nerve (VIII) has a special 

 somatic sensory nature. The large ganglion of this nerve 

 lies between the otic capsule and the brain; it has vestibular 

 and cochlear divisions. The ganglion receives nerves from the 

 various parts of the inner ear. The neurons of the ganglion 

 are unique in being bipolar, like those of the retina, the 

 nasal epithelium, or the early stages of spinal sensory gan- 

 glion. 



The glossopharyngeal nerve (IX) is associated with the 

 third arch, the first branchial arch. It has motor and sen- 

 sory components. The sensory fibers are from the posterior 

 part of the tongue and the pharyngeal area; the root gan- 

 glion is the petrosal. The motor component serves the muscu- 



386 



THE NERVOUS SYSTEM 



