From the optic lobes the two trunks pass forward along the under surface of the cerebri, forming the optic chiasm at the hypophysis (Fig. 75, No. _C_, 15), at which point the nerve fibers originating from the right side pass to the left, and _vice versǎ_. From the chiasm the true optic nerves pass forward to the posterior surface of the eyeball. They are composed of a bundle of very fine, marrow-like nerves. These fibers enter into the ganglionic cells of the retina. By removal of the finely adherent neurilemma, the optic nerve is seen to be composed of parallel, longitudinal lamellæ, the margins of which are mostly free on one side. =The Motor Oculi.=—The motor oculi (Fig. 62, No. _C_, 10), the third cranial nerve is a motor nerve. It originates close to the base of the brain behind the position of the hippocampus of mammals on the inner side of the crus cerebri, and also on the inner somatic column close below and somewhat aside from the aqueduct of Sylvius. The nerve leaves the brain cavity through a distinct foramen near the foramen opticum, and supplies the following eye muscles: first, after entering the orbit, it sends a branch upward to the inferior portion of the inferior rectus; then, it gives off the thick ramus ciliaris. The trunk then extends under the optic nerve and passes forward to innervate the inferior rectus, the internal rectus, and the inferior oblique. A ciliary ganglion forms on the trunk. =The Patheticus.=—(Fig. 62, No. _C_, 11) is a small motor nerve, originating close to the sulcus centralis, in the circle of the center brain over the valve of Vieussens, between the posteriors of the optic lobes. It extends in a dorsal direction between the cerebellum and the lobus opticus, to the posterior of the latter of which it then forms a loop ventrally. Lying close to the optic foramen it passes through a fine opening into the eye cavity and supplies the superior oblique muscle of the eye. During its course it passes dorsally over the optic nerve, and then crosses dorsally over the ophthalmic division of the fifth pair of cranial nerves and the internal rectus muscle. =The Trifacialis.=—(Fig. 62, No. _C_, 12). This, the fifth cranial, is a mixed nerve and is divided into two parts, portio major and portio minor. The _portio major_ originates in the ganglion cells of the posterior part of the brain. Commencing near the medulla oblongata it passes through the posterior part of the brain, then upward and outward; and along its route it forms the Gasserian ganglion, which lies partly in the cranial cavity or in its wall. The _portio minor_ consists of the downward passing fibers containing the motoric elements which are distributed to the muscles of the jaw and of the eye. The roots are found in the ganglion from the center and back brain, close below the pathetic nerve origin, where the pathetic passes between the lobus opticus and the pars peduncularis; it then takes a lateral course between the two.