Feline Ophthalmology – The Manual
Anatomy and function The eyelids develop from surface ectoderm and join to fuse along the future palpebral fissure. They remain fused until 10-14 days after birth. In the adult cat, the palpebral fissure measures approximately 28 mm in length, although there is some variation with brachycephalic cats tending to have slightly longer eyelids. The eyelids are composed of an outer layer of skin, a supportive tarsal plate, smooth and striated muscle and an inner conjunctival lining (Fig. 1). The upper and lower eyelid of the cat lack true cilia (eyelashes) but, in the upper eyelid, the first row of hairs fulfils much the same function. The tarsal plate of each eyelid contains approximately 30 lipid-secreting meibomian glands whose openings form a distinct groove along the eyelid margin (also referred to as the ‘grey line’). The relatively thin, striated orbicularis oculi muscle is closely attached to the overlying skin and completely encircles the palpebral fissure. This muscle is responsible for eyelid closure and is innervated by the palpebral branch of the facial nerve (CN VII). As with most mammals, the upper eyelid of the cat is more mobile than the lower. The main elevator of the upper eyelid is the striated levator palpebrae superioris muscle, which is innervated by the oculomotor nerve (CN III), and the smooth Müller’s muscle, which is sympathetically innervated. Lower eyelid depression is performed by the malaris muscle which is innervated by the dorsal buccal branch of the facial nerve. The main function of the eyelids is to protect and maintain the health of the ocular surface. To do this effectively they need to be in close apposition with the cornea and to be able to meet completely during blinking. The nictitans (syn. nictitating membrane, third eyelid) consists of a T-shaped cartilage covered on both the palpebral and bulbar surfaces with conjunctiva (Fig. 2). The nictitans gland is located at the base of the cartilage and contributes significantly to tear production, producing an estimated 30-50 % of the total aqueous tear volume. The lacrimal system has both secretory and excretory components (Fig. 3). The secretory component consists of the various glands that contribute to the preocular tear film. The tear film of the cat is approximately 7 µm thick and is made up of three layers.
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Figure 1. Eyelid anatomy.
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Figure 2. Nictitans anatomy.
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