Eye Anatomy and Physiology

Orbit--Orbit refers to the bony socket of the skull that contains the eyeball or globe, and the surrounding nerves, blood vessels, fat, connective tissue, and muscle. The equine orbit is a large, conical cavity protected by a complete bony rim. Respiratory sinuses border the horse orbit on the midline (center of the head).

Animals with laterally positioned orbits (one on each side of the head), such as horses, have tremendous peripheral vision but reduced binocular vision and depth perception.

Eyelids--Anatomically, the eyelids are divided into the skin, a fast-acting and powerful muscle (for rapid blinking), and an inner lining of palpebral conjunctiva. The eyelids are thin and contain many blood vessels. Eyelids protect the eye, produce and distribute the tears, aid tear drainage, and help control the amount of light entering the eye. Eyelid swelling can be an early sign of corneal ulceration (more about corneas in a moment). Meibomian glands, which secrete the fatty component of the tear film that lubricates the eye, are found at the eyelid margins. Horses have large eyelashes or cilia on their upper eyelid margin and none on the lower lid. Long sensory hairs or vibrissae provide sensation to the skin surrounding the eyes of horses.

Source: TheHorse