III. N. OCULOMOTORIUS.--The third or oculomotor nerve arises (Fig. 138, _III_) from the pedunculus cerebri and passes into the orbit through the orbital fissure. It passes between the lateral and superior recti, sends a large branch to the superior rectus, supplies the medial rectus and the retractor oculi, passes laterad of the optic nerve and supplies the inferior rectus, and sends a long branch to the inferior oblique (Fig. 166, _f_). At the point where the branch is given off to the inferior oblique muscle is the triangular reddish =ciliary= (or ophthalmic) =ganglion=, about two millimeters in diameter. It is directly attached to the inferior oblique branch of the third nerve and receives a delicate sensory filament from the ophthalmic nerve. From the ciliary ganglion proceed two =short ciliary nerves= which pass along each side of the optic nerve to the eyeball. These nerves penetrate the sclerotic at the sides of the eyeball to be distributed to the ball. IV. N. TROCHLEARIS.--The fourth nerve, the trochlearis (or patheticus), arises from the lateral border of the velum medullare anterius, as already described (Fig. 141, _n_). It passes craniad, emerges through the orbital fissure in company with the oculomotor, abducens, and ophthalmic, passes dorsad of the superior rectus muscle, and reaches (Fig. 154, _l_) the caudal border of the belly of the superior oblique muscle near its middle. It supplies only the superior oblique. V. N. TRIGEMINUS.--The fifth nerve, the trigeminus (or trifacial) arises (Fig. 138, _V_) by two roots, a large sensory and a small motor root (4), as already described (p. 347). One of these, the motor (4), is smaller and more ventral; the other, the sensory root, is larger and dorsal. The dorsal root soon enlarges to form a large ganglion, the =semilunar= (or Gasserian) =ganglion= (Fig. 138, _k_), from which three branches diverge. One branch is joined by the ventral root (4), which passes over the ventral surface of the semilunar ganglion; and the nerve thus formed is the =mandibular= division (1) of the fifth nerve. Of the other two branches from the ganglion, the middle and longest is the =maxillary= nerve (2), and the smallest is the =ophthalmic= (3). The mandibular nerve is thus mixed, motor and sensory, while the others are sensory. 1. =N. ophthalmicus.=--The ophthalmic or first division of the fifth nerve arises from the semilunar (or Gasserian) ganglion. It passes out of the cranial cavity and into the orbit by way of the orbital fissure, in company with the third, fourth, and sixth nerves and with the extension of the carotid (arterial) plexus. It passes between the superior and medial recti along with the third nerve, crosses dorsad of the optic nerve, and divides into =infratrochlear= and =ethmoidal= branches. In the orbital fissure it gives off the =frontal= nerve, and while crossing the optic it gives off the =long ciliary= nerve.