THE NERVOUS SYSTEM 527 



brain wall. Like the trochlear, the optic nerve has a chiasma which lies 

 below the brain immediately anterior to the infundibulum. Half its 

 fibers enter the chiasma and cross to the opposite side of the brain, the 

 other half effect central connexions with the thalamus and optic lobe 

 of the side from which they enter. In lower vertebrates, the central 

 connexions of the optic are with the optic lobes of the mesen- 

 cephalon. But in mammals they are shifted mostly to the occipital 

 lobes of the hemispheres, which they reach indirectly by way of the lateral 

 geniculate body of the thalamus. 



III. Oculomotorius. The motor nucleus of the oculomotor lies in the 

 somatic motor column at the base of the midbrain, median to the cerebral 

 peduncle. It contains somatic efferent and visceral efferent fibers. It is 

 an eye muscle nerve innervating four eye muscles; it also innervates the 

 ciliary muscle within the eye by way of the ciliary ganglion, a para- 

 sympathetic ganglion attached to the oculomotor and ophthalmic nerves. 

 Like the other eye muscle nerves the oculomotor nerve remains relatively 

 unchanged from fishes to man. 



IV. Trochlearis. The trochlear, another eye muscle nerve, has its 

 nucleus in the base of the metencephalon, and its root of origin from the 

 dorsal constriction which separates the optic lobes from the meten- 

 cephalon. In cyclostomes its root is said to be connected with the lateral 

 wall of the medulla between those of the ophthalmicus and trigeminal 

 nerves, the roots of which are separate in cyclostomes. In elasmobranchs 

 its fibers form a chiasma over the isthmus behind the midbrain. In 

 mammals and man its chiasma lies posterior to the inferior colliculus. 

 Although in having a chiasma, the trochlearis is unique among motor 

 nerves, we must class it as a somatic motor nerve, since its fibers are 

 connected with cells in the somatic motor column of the brain posterior to 

 those of the oculomotor. Moreover, the muscle which it innervates 

 develops from a mesodermal somite (somite 2). 



V. Trigeminalis. In most vertebrates the trigeminal arises from the 

 lateral wall of the oblongata by two roots, the larger sensory, the smaller 

 motor. Its sensory ganglion, the Gasserian or semilunar, sends an 

 ophthalmic branch to the skin of the forehead, a maxillary branch to the 

 upper lip and the teeth of the upper jaw, and a mixed mandibular branch 

 to the teeth of the lower jaw and to the tongue. In man and mammals all 

 three branches have somatic afferent fibers. (Fig. 438) 



The motor root is that of the masticator nerve, which supplies special 

 visceral efferent fibers to the chewing muscles. In man, the nucleus of 

 the masticator nerve lies in the visceral motor column in the wall of the 

 oblongata, median to the pons. General visceral or proprioceptive fibers 

 enter the semilunar ganglion from the chewing muscles, and pass to the 

 mesencephalic nucleus in the roof of the midbrain. Such sensory fibers 



