Eyes

Bulbus oculi

Eyeballs housed in complete bony orbits on the sides of the head. The horse has one of the largest eyes of any land mammal, with a horizontal pupil and iridic granules on its margin. This model shows the visible surface of the eye, not its internal structure.

Laterality:: Paired (left and right)

Draft, not yet reviewed by a veterinarian. Content may contain errors. It is published so the community can review it.

View piece alone in 3D

This piece is a schematic shape, not a scan: there is no freely licensed model for this structure yet.

3D model of this piece: "Equine Model" by Lova Dahn (Sketchfab), CC BY 4.0

Overview

The eye (oculus) consists of the eyeball (bulbus oculi) and its accessory organs: eyelids, conjunctiva, lacrimal apparatus and extrinsic muscles. In the horse the eyeball is very large and sits on the side of the head in an orbit closed by bone. This position gives a very wide visual field, covering almost all its surroundings, with a narrow zone of binocular vision in front and blind spots just in front of the muzzle and behind the body.

What is visible from outside

Cornea (cornea): transparent anterior part of the fibrous tunic, horizontally oval.
Pupil: horizontal and elliptical in bright light.
Iridic granules (granula iridica): pigmented masses on the pupillary margin, mainly the upper one; they shade the pupil.
Sclera (sclera): white part of the fibrous tunic; little of it shows except at the lateral angle or in poorly pigmented eyes.
Eyelids (palpebrae): with lashes on the upper lid and long tactile hairs around them.
Third eyelid (plica semilunaris conjunctivae): a conjunctival fold with a cartilage at the medial angle; it slides across the cornea when the globe is retracted.
Nasal opening of the nasolacrimal duct: visible on the floor of the nasal vestibule, near the junction of skin and mucosa; the duct can be flushed through it.

Clinical relevance

Corneal ulcer: common because of the eye's prominent position; confirmed with fluorescein. In the horse it can become infected by fungi and worsen quickly.
Equine recurrent uveitis: repeated episodes of inflammation of the uvea; it is the most common cause of blindness in the horse. It has been associated with Leptospira infection.
Squamous cell carcinoma: the most common ocular tumour; it affects the third eyelid, eyelids and limbus, mainly in horses with little pigment around the eye.
Entropion in foals: common in weak or dehydrated newborns; the lower lid margin rolls inwards and the hair irritates the cornea.
Examination: blocking the auriculopalpebral nerve stops the horse from squeezing its eyelids shut and makes examining and treating the eye easier.