Palpebrae comes from the Latin word palpitare, which means "to flutter." In ophthalmology, the palpebrae, more commonly known as the eyelids, are protective layers of skin that surround the anterior surface of the eyes. Their basic function is to prevent the eyes from being injured by the entry of foreign materials like dust and debris, or the assault of bright lights that could permanently ruin the eyes.
The eyelids are also useful in keeping the eyes well lubricated by producing and spreading tears and mucus evenly across the eyeballs. The palpebrae for each eye are composed of two lids: an upper eyelid that extends upwards from the eye towards the eyebrow, and a lower eyelid that descends from the eye towards the cheek. The anatomy of the eyelid includes important structures such as the skin, hypodermis, levator palpebrae muscle, orbicularis oculi muscle, orbital septum, tarsal plates, and conjunctiva.
The skin of the palpebrae is less than 0.04 inches (1 mm) thick, making it by far the thinnest skin in the human body. This skin contains pigment cells, sweat glands, and fine hairs called eyelashes. These eyelashes block dirt and dust from entering the eyes. Beneath the layer of skin is the hypodermis. Unlike in other parts of the body, the hypodermis of the palpebrae scarcely contains fat, but mainly consists of loose connective tissue.
There are two muscles that enable the palpebrae to keep true to their Latin definition. These are the levator palpebrae and orbicularis oculi muscles, which are responsible for the opening and closing of the eyelids. The former muscle is located deep within the eye sockets and extends over the eyeballs, where it is linked to the eyelids via the levator aponeurosis tendon, the tissue that retracts and lifts the upper eyelid to open the eyes. The latter muscle is all around the eyeballs, and is the only muscle that controls the closing of the eyelids. Any damage to this muscle can result in the potential loss of an eye.
While the palpebrae protect the anterior surface of the eyes, the orbital septum shields their posterior area. A structure of connective tissue, the orbital septum forms a layer of fat surrounding the upper and lower rims of the eyeballs. It is connected to the levator palpebrae muscle in the upper eyelids and the tarsal plates in the lower eyelids, somewhat influencing movement of the eyeballs.
Tarsal plates are thick fibrous tissue that stretches across the eyelids, giving them their unique shape. These plates consist of two tarsi: the superior tarsus in the upper eyelid and the inferior tarsus in the lower eyelid. Each tarsus has a maximum length of about 1.14 inches (29 mm) and a width of 0.04 inches (about 1 mm). The larger of the two, the crescent-shaped superior tarsus, has a vertical height of approximately 0.4 inches (10 mm) in the center of the eye. Its counterpart, the inferior tarsus, is oval-shaped and is smaller in vertical height. Both these tarsal plates contain anywhere between 20 to 50 Meibomian glands, which are responsible for the dissemination of sebum in the eyes.
The conjunctiva is a thin and clear layer of mucous membrane that lines the insides of the palpebrae and covers each eyeball. Its role is to produce optical fluids, such as mucus and tears. In spreading these fluids all over the eyes, the conjunctiva prevents the entry of bacteria and foreign matter. When the eyes are exposed to very aggressive materials, the conjunctiva can become red and inflamed, resulting in the eye disease called conjunctivitis or pinkeye.
While discussing the anatomy of an eye, the palpebrae are often defined rather simplistically. Due to their protective nature, however, they are the eyes' first line of defense against any injury and bacterial infection that could permanently harm the organ of sight.