—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.
a. N. frontalis.—The frontal nerve passes along the lateral border of the superior oblique muscle and then laterad of the pulley to near the middle of the supraorbital crest of the frontal bone. Here it passes out of the orbit and is distributed to the integument of the upper eyelid and the adjacent region at the side of the nose.
b. N. infratrochlearis.—The infratrochlear nerve passes between the superior rectus and the superior oblique in the first part of its course. It then passes ventrad of the superior oblique and ventrad of the pulley to be distributed to the integument of the upper eyelid near the inner angle.
c. N. ethmoidalis.—The ethmoidal nerve passes along with the ethmoidal artery through the ethmoidal foramen (or foramina) in the orbital plate of the frontal bone. It is finally distributed to the mucosa of the nose and to the cartilage and integument of the snout.
d. N. ciliaris longus.—The long ciliary nerve arises from the ophthalmic and passes along the optic nerve to be distributed to the eyeball. It divides into several branches before penetrating the sclerotic.
One or two small communicating branches to the ciliary ganglion are given off at about the same point as the long ciliary nerve. (For a description of this ganglion see the account of the oculomotor nerve, [page 369].)