Overview
The eye (oculus) consists of the eyeball (bulbus oculi) and its accessory organs: eyelids, conjunctiva, lacrimal apparatus and extrinsic muscles. The cat's eyes are very large relative to the skull and face forward, giving a wide binocular field that helps to judge distance when hunting. The orbit is not closed by bone: the orbital ligament completes the ring.
What is visible from outside
- Cornea: transparent and very large; it covers almost the whole visible surface of the eye, so the sclera barely shows.
- Iris and pupil: the pupil constricts to a vertical slit in bright light and dilates until almost round in the dark. This shape allows a very wide range in the amount of light entering the eye.
- Eyelids (palpebrae): skin folds with tarsal glands; the cat has no true eyelashes.
- Third eyelid (plica semilunaris conjunctivae): a conjunctival fold at the medial angle with a T-shaped cartilage and a gland that supplies part of the tear film.
Inside the eye
- Tapetum lucidum: a reflective cellular layer of the choroid in the dorsal half of the fundus, yellow-green in colour. It sends light back through the retina and improves vision in dim light; it causes the cat's eyeshine in torchlight or a flash.
- Retina: rich in rods, adapted to low-light vision.
Clinical relevance
- Feline herpesvirus keratoconjunctivitis (FHV-1): causes conjunctivitis and dendritic corneal ulcers, recurring with stress.
- Corneal ulcer: confirmed with fluorescein. In brachycephalic cats it can progress to a corneal sequestrum, a brown or black necrotic plaque typical of the cat.
- Entropion: common at the medial angle of the lower eyelid in Persians.
- Feline infectious peritonitis: the dry form causes uveitis and retinal lesions.
- Hypertensive retinopathy: systemic hypertension, associated with chronic kidney disease and hyperthyroidism, causes retinal haemorrhage and detachment with sudden blindness and dilated pupils.
- Taurine-deficiency retinal degeneration: in cats fed inadequate diets.
- Bilateral protrusion of the third eyelid: may signal systemic or gastrointestinal illness; if unilateral, Horner's syndrome.