The functions of the eyelid are protection of the eye, distribution of tears, mechanical cleaning of the ocular surface, regulation of light exposure for the eye, and partial production of the tear film. The eyelid is composed of skin, subcutaneous connective tissue, fibrous tissue (tarsus), muscle, and mucous membrane (palpebral conjunctiva). The eyelid skin varies in thickness, from 0.5 mm at the eyelid margin to 1 mm at the orbital rim. Except for fine vellus hairs, the eyelashes (cilia) are the only hairs of the eyelids and are twice as numerous along the upper eyelid compared with the lower margin. Eyelashes are replaced every 3–5 months and usually regrow in 2 weeks when cut and within 2 months if epilated. They catch small particles and also work as sensors to stimulate reflex eyelid closure. Blinking stimulates the lacrimal pump to release tears, which are then spread across the cornea, flushing foreign material. Most individuals blink an average of 10–15 times per minute at rest, 20 times per minute or more during a conversation, and as few as 5 times per minute when concentrating (eg, reading). Blink frequency has also been shown to change in different positions of gaze. The orbicularis oculi muscle, which is innervated by cranial nerve (CN) VII, closes the upper and lower eyelids (Fig 1-1). The levator palpebrae muscle, innervated by CN III, inserts into the tarsal plate and skin and elevates the upper eyelid. The Müller muscle, innervated by sympathetic nerves, increases the width of the palpebral fissure.
The epidermis of the eyelids abruptly changes from keratinized to nonkeratinized stratified squamous epithelium at the mucocutaneous junction of the eyelid margin, along the row of meibomian gland orifices. Holocrine sebaceous glands and eccrine sweat glands are present in the eyelid skin. Near the eyelid margin are apocrine sweat glands (the glands of Moll) and numerous modified sebaceous glands (the glands of Zeis).
Figure 1-1 Cross section of the upper eyelid.
(Illustration by Christine Gralapp.)
See BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 7, Oculofacial Plastic and Orbital Surgery, for additional discussion of eyelid anatomy.
Argilés M, Cardona G, Pérez-Cabré E, Rodríguez M. Blink rate and incomplete blinks in six different controlled hard-copy and electronic reading conditions. Invest Ophthalmol Vis Sci. 2015;56(11):6679–6685.
Lin LK, Gokoffski KK. Eyelids and the corneal surface. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:40–45.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.