Equine ocular anatomy and ophthalmic examination
Susan M. Carastro, DVM, MS
Animal Eye Specialty Clinic, 3421 Forest Hill Boulevard, West Palm Beach, FL 33406, USA
This article is intended to provide the practitioner with a succinct but complete source regarding equine orbital and ocular anatomy, instrumentation available for ophthalmic examination, amethodical examination technique, sedation and regional nerve blocks, and diagnostic procedures involving the eye. Such knowledge of orbital and ocular anatomy is essential to allow recognition of normal, normal variations, or an abnormality of the equine eye and orbit.
Anatomy Equine orbit The equine orbit is closed laterally and completely encircles the globe, with a bony rim. The bones thatcomprise the orbit are the frontal, lacrimal, zygomatic, temporal, palatine, and sphenoid. The orbital rim consists of the frontal bone superiorly/nasally containing the supraorbital foramen, lacrimal bone nasally, zygomatic bone inferiorly, and zygomatic process of the temporal bone completing the orbit temporally. The palatine and sphenoid bones form the medial wall of the orbit. The orbital ﬂooris soft tissue consisting mostly of fat and resting on the pterygoid muscles. Approximate dimensions of the orbit are 62 mm wide Â 59 mm high Â 98 mm deep  but may vary depending on the breed and size of the horse. Approximate globe dimensions are 42 to 44 mm from the anterior to posterior axis, 45 to 50 mm vertically, and 50 to 54 mm horizontally . The orbital space contains the globe, thirdeyelid, extraocular muscles, nerves, vessels, lacrimal gland, and fat. Foramina within the orbital bones allow vessels and nerves to traverse from the cranial cavity to the orbit. The alar or rostral foramen contains the
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S.M. Carastro / Vet ClinEquine 20 (2004) 285–299
maxillary artery and nerve. The ethmoidal foramen permits passage of the ethmoidal vessels and nerve. The oculomotor (cranial nerve III), trochlear nerve (cranial nerve IV), ophthalmic branch of the trigeminal nerve (cranial nerve V), and abducens (cranial nerve VI) travel through the orbital foramen to reach the orbit. The optic nerve (cranial nerve II) and internalophthalmic artery enter the orbit through the optic foramen. The extraocular muscles facilitate movement of the globe. The rectus muscles include the superior, inferior, medial, and lateral recti, which tether the globe within the orbit and are capable of moving the globe in their respective directions. The superior oblique attaches to the superior/lateral aspect of the globe, rotating the superiorportion of the globe inferiorly and nasally. The inferior oblique muscle attaches to the inferior/lateral aspect of the globe, moving the globe superiorly and nasally. The retractor bulbi muscle consists of four muscle bellies withdrawing the globe posteriorly. The retractor bulbi muscle is one of the largest extraocular muscles, or at least it seems so when evaluating a horse with a painful eye.Origination of the muscle is from the posterior orbital wall, and insertion is to the posterior sclera. Innervation of the superior, inferior, medial rectus, and inferior oblique muscles is via the oculomotor nerve. The superior oblique is innervated by the trochlear nerve and the lateral rectus, and retractor bulbi are innervated by the abducens nerve. Eyelids The eyelids comprise haired skin,subcutaneous tissue, the tarsal plate, the orbicularis oculi muscle, the levator palpebrae superioris muscle, Mueller’s muscle, and the palpebral conjunctiva from exterior to interior. Specialized structures in the lid margin are the meibomian glands and glands of Zeiss. The tarsal plate is an area of dense connective tissue lending rigidity to the eyelid margins. The meibomian glands are oriented...