The eye and the seven associated extraocular muscles reside in the orbit and are cushioned by periorbital fat. The orbit may be closed (completely surrounded by bone) or open (composed of bone and connective tissue) depending on the species (see Table 12.1). The eyelids protect the anterior surface of the eye. They are covered with haired skin on the outer surface and the conjunctiva, a thin mucous membrane, on the inner surface. Eyelids sweep foreign material from the anterior surface of the eye and facilitate movement of tear film to the nasolacrimal drainage channels at the medial canthus. Cilia (eyelashes) are most numerous in the upper lid and are commonly lacking in the lower lid. The nictitans, or third eyelid, resides at the medial angle of the palpebral fissure. The nictitans contains cartilage, and the seromucoid nictitans gland is molded to the base of the cartilage. The free edge of the third eyelid may be pigmented or unpigmented.

Parts of the eye are illustrated in Figure 12.1. The globe is formed by three concentric coats. The fibrous outer tunic gives the eye its shape and consists of the cornea and the sclera. The cornea is clear and refracts light. The sclera is opaque and provides the site of attachment for the extraocular muscles. The sclera is separated from the cornea at the limbus. The innermost tunic is the retina. The optic nerve (cranial nerve II) is an extension of the brain and is surrounded by meninges.

The thick middle vascular tunic, also referred to as the uvea, contains the choroid, ciliary body, and iris. The choroid envelopes the posterior of the globe, except at the optic nerve head, and is continuous with the ciliary body anteriorly. The ciliary body lies between the choroid and iris. The posterior flat portion is the pars plana, and the raised anterior portion is the pars ciliaris. Zonular fibers arise from the pars ciliaris and suspend the lens behind the pupil. The ciliary body also produces aqueous humor. The iris is the most anterior portion of the uvea and is a thin, circular diaphragm with a central pupil, which varies from horizontal to round to vertical in domestic mammals (see Table 12.1). In many hooved animals, the iris has a specialized structure (corpora nigra) on the dorsal edge that is thought to act like a sunshade (Figure 12.2). The anterior chamber is located between the cornea and iris, and the posterior chamber is between the iris and the lens; both of these chambers are filled with aqueous humor. Aqueous humor drains through the filtration angle at the base of the iris in the anterior chamber.

The innermost layer of the eye is the neural tunic or the retina. The retina is the location of the photosensitive cells that detect light. The vitreous body is a gelatinous mass located in the posterior segment of the eye, between the lens and the retina. The vitreous body helps keep the retina in place. The tapetum lucidum is a reflective layer behind the retina that is responsible for the eyeshine seen when a beam of light is directed into the eye. In domestic animals, the tapetum can be composed of either rectangular cells (cellular) or collagen bundles (fibrous; see Table 12.1).

Ocular anatomy is complex, and the reader is referred to standard reference texts for more detail (see References).

The ear is divided into three parts: the external ear, the middle ear, and the inner ear (Figure 12.3). The external ear consists of the pinna (ear flap) and the external auditory meatus, which has a vertical and horizontal component. The auricular cartilage forms the pinna and most of the ear canal. The shape of the pinna is species and breed specific. A complex group of skeletal muscles, all innervated by the facial nerve, control pinnal movements. Haired skin covers the convex, and concave surfaces of the pinna.

The conical opening into the external auditory meatus is formed by elastic cartilages that funnel sound waves into the ear canal. This conical opening gradually narrows into the vertical ear canal that runs ventrally and slightly rostrally. The vertical canal bends medially and continues as the horizontal ear canal. The horizontal ear canal is surrounded by and merges into the osseous acoustic meatus near the skull. Hair follicle numbers within the external ear canal decrease toward the tympanic membrane.

The tympanic membrane separates the external and middle ears. The external surface of the tympanic membrane is covered by a thin layer of squamous epithelium that overlies a thin stromal layer. The dorsal part is termed the pars flaccida and is relatively flaccid and very vascular. The large pars tensa is located ventrally and is normally translucent.

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