THE NERVE SUPPLY OF THE EYE
The nerve supply to the eye and its accessory structures is derived from no fewer than six cranial nerves. Most of these enter the orbital cone, but some reach accessory structures directly.
The optic nerve (II) enters the orbit through the optic foramen and passes to the light receptor cells in the retina. It is rather slack in order to allow for the movements of the eye and is covered by meninges that it acquired during its development as the stalk of the optic cup.
Though the name of the oculomotor nerve (III) implies that it controls movement of the eyeball, it does not innervate all the ocular muscles. It enters the orbit through the orbital foramen (fissure; foramen orbitoro- tundum in ruminants and the pig) and sends branches to the levator palpebrae; the dorsal, medial, and ventral recti; the ventral oblique; and part of the retractor muscles.
The trochlear nerve (IV) accompanies the third nerve and innervates the dorsal oblique muscle.
The ophthalmic and maxillary divisions of the trigeminal nerve (V) send branches to the eye. The ophthalmic nerve passes through the orbital foramen and supplies the following sensory branches: long ciliary nerves to the eyeball, especially the cornea; a lacrimal nerve to the eyelids and conjunctiva of the lateral angle; a supraorbital nerve that accompanies the like-named artery through the supraorbital foramen to supply the upper eyelid and skin medial to the orbit; an infratroch- lear nerve (not present in all species) sensory to structures near the medial angle of the eye; and an ethmoidal nerve that follows the ethmoidal artery to innervate the caudal part of the nasal cavity. The maxillary nerve has only one relevant branch; this, the zygomatic nerve, supplies the lateroventral segment of the eyelids and conjunctiva by a zygomaticofacial branch and skin caudal to the orbit by a zygomaticotemporal branch. In horned cattle the zygomaticotemporal branch furnishes the clinically important cornual nerve to the horn.
These sensory nerves to the orbit provide the afferent limbs of the palpebral and corneal reflexes that stimulate the orbicularis oculi to close the eye when the lids or cornea are touched.The abducent nerve (VI) enters through the orbital foramen. It innervates most of the retractor bulbi and the lateral rectus muscles.
The auriculopalpebral branch of the facial nerve (VII) passes between the eye and ear and thus approaches the eyelids from behind. It innervates the orbicularis oculi. It may be blocked to immobilize the lids or to relieve the “pressure” that the tonus of the muscle may exert on a painful globe. (The levator palpebrae is not immobilized by this block.)
Sympathetic nerve fibers arising from the cranial cervical ganglion follow arteries or the ophthalmic nerve to the orbit, where they innervate the orbital muscle and dilator of the pupil. Tonus in the orbital muscle keeps the eyeball protruded, the third eyelid retracted, and the palpebral fissure open. Loss of sympathetic innervation results in a sunken eye, protrusion of the third eyelid, and constriction of the pupil (Horner’s syndrome). Dilation of the pupil (mydriasis) is initiated by fear, excitement, or pain.
Parasympathetic presynaptic nerve fibers enter the orbit within the oculomotor nerve. They synapse in the ciliary ganglion, and the postsynaptic fibers (which form the short ciliary nerves) innervate the ciliary muscle and the constrictor of the pupil. They control both the accommodation of the lens and the pupillary contraction (miosis) response to light.