Introduction: Anatomy of the Lips and Eye
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The eyelids are highly specialized structures with peculiar anatomic components. The ocular globes are allocated in two symmetrically bony cavities called orbits, consisting of seven bones that develop the orbital walls. The roof is composed mostly of the orbital plate of the frontal bone and posteriorly of a minor part of the sphenoid bone. The lateral wall comprises the orbital surface of the zygomatic bone and the sphenoid bone. The floor is composed of the orbital plate of the maxilla anterolaterally of the zygomatic bone and posteriorly of the palatine bone. The medial wall consists of the ethmoid, frontal, lacrimal, and sphenoid bone. The eyelid skin, which is less than 1 mm thick, is a thin epidermis constructed from a stratified epithelium of 67 cell layers. The dermis contains elastic fibers, blood vessels, lymphatics, and nerves. The underlying fat is scant or not present in the subcutaneous tissue, where the hair follicles and pilosebaceous glands are located. The apocrine glands of Moll are located near the lid margin, and the sebaceous glands of Zeiss are associated with the follicles of the eyelashes. The eyelids function to protect the eye globe from local and external injuries. Furthermore, they regulate the light that reaches the eye and uniformly distribute the tear film, mucus, and oil during blinking, of great importance for the health of the cornea. The eyelids are divided into upper and lower eyelids, which are similar but with different characteristics mainly in the lid retractor arrangement. The space between the open lids is known as the palpebral fissure, which measures 7–12 mm, while the normal excursion of the lids is 14–17 mm. In the normal adult fissure, the highest point of the upper lid is just nasal to the center of the pupil, while the lowest point of the lower lid is just temporal to the center of the pupil. In youths, the upper lid margin rests at the upper limbus, whereas in adults it rests 1.5 mm below the limbus. The lower eyelid margin rests at the level of the lower limbus. The lateral canthal angle is 2 mm higher than the medial canthal angle in Europeans, but is 3 mm higher in Asians. The distance from the medial canthus to the midline of the nose is approximately 15 mm. The lateral canthus lies directly over the sclera, and the medial canthus is separated from the eye by the lacrimal lake and caruncle, a yellowish tissue containing sebaceous and sweat glands. The lid margins are 2 mm wide and form the junction between the skin and the conjunctiva, the mucous membrane of the lids. They meet at the gray line, near the posterior edge of the lid margin, the junction of the anterior and posterior lamellae of the lids. The eyelashes are located anteriorly and the openings of the meibomian glands posteriorly. There are approximately 100–150 eyelashes on the upper lid and about 50–75 on the lower. The follicular structure of eyelashes includes the sebaceous (Zeiss) and sweat (Moll) glands, while the tarsal glands (Meibomian) open posteriorly to the lid margin. The tears that appear at the tips of the small papillae are drained from the surface of the eyes through the openings by a pump mechanism. The lacrimal secretory system controls the amount of tears and is divided into the basic and reflex secretors. The basic secretor is composed of three sets of glands. (1) Conjunctival, tarsal, and limbal mucin-secreting goblet cells; the overlying aqueous layer is spread more uniformly because of this inner layer (precorneal tear film). (2) The accessory lacrimal exocrine glands of Krause and Wolfring, located in the subconjunctival tissue. (3) The oil-producing Meibomian glands and the palpebral glands of Zeiss and Moll. The reflex secretor is divided into two parts by the lateral horn of the levator palpebrae superioris.