The eye, the organ of vision, consists of the eyeball and various adnexa—accessory structures such as the ocular muscles that move the eyeball, the lids that protect it, and the lacrimal apparatus that keeps its exposed parts moist.

Most of the adnexa are housed in the orbit, where the eyeball is embedded in generous quantities of fat. The eyelids arise from the bony margins of the orbit and, like curtains, are intermittently drawn over the exposed part of the eye (blinking) to distribute the tears or lacrimal fluid for protection; they are kept across the eye during sleep, when vision is not required.

The eyes of the domestic mammals protrude more from the surface of the face than those of primates, ourselves included. Their position in the head is related to the animal's environment, habits, and method of feeding. In general, predatory species (cat, dog) have eyes set well forward, to provide a large field of binocular vision that allows for focus on near objects and for the perception of depth (Fig. 9.1). Prey species (herbivores: horses, ruminants, rabbits) carry their eyes more laterally so that the right and left fields of vision hardly overlap; consequently, these animals have a large field of vision but little capacity for binocular vision.

When an animal is emaciated, the orbital fat is reduced and the eyes sink within the orbits, giving the face a gaunt appearance.