The Nerve Supply to the Eye of the Yak (Bos Grunniens) in the Qinghai-Tibetan Plateau

Fifteen head specimens of the yak (Bos grunniens) were dissected to study the anatomical characteristics of the nerves supplied to a yak's eye. In the orbit, the optic nerve was “S” shaped and received 1–2 twigs from the ciliary ganglion. The ciliary ganglion was a grayish fusiform structure, located between the ventral branch of oculomotor nerve and retractor bulbi muscle, which gave off 2–3 long ciliary nerves—one penetrated the sclera, and the others penetrated into the optic nerve. In addition, there were 3–4 communicating branches from the nasociliary nerve and the ventral branch of the oculomotor nerve. The oculomotor nerve divided into dorsal and ventral branches; the dorsal branch divided into 4–6 twigs, two of which supplied the rectus dorsalis muscle and the others supplied levator palpebra superioris. The ventral branch furnished 3–5 twigs to the rectus medialis and ventralis muscles. After it received a branch from the maxillary nerve, it finally supplied the obliquus ventralis muscle. The trochlear nerve supplied the obliquus dorsalis muscle. The ophthalmic nerve divided into zygomaticotemporal, frontal sinus, and nasociliary nerves. After the zygomaticotemporal branch received a branch of maxillary nerve, it supplied the skin of the temporal region, the lacrimal gland and the upper eyelid; the frontal nerve supplied the lateral portion of upper eyelid; the nasociliary nerve divided into ethmoidal and infratrochlear nerves. The ethmoidal nerve penetrated the ethmoidal foramen, and the infratrochlear nerve supplied the upper eyelid and skin of the frontal region. The abducent nerve supplied the rectus lateral and retractor bulbi muscles. Therefore, the yak does not have accessory ciliary ganglion.