The eyelids are thin, mobile folds of skin that cover the eyeball. They provide protection from external elements such as excessive light or injury, and prevent the eyes from drying out by regulating the flow of tears across the surface of the eye. This article will discuss the anatomy of the eyelids, including their layers, vasculature and innervation.
The eyelid contains five main layers, ranging from superficial to deep.
A stye (or hordeolum) is an infection of a hair follicle or Meibomian gland around the eyelash. If the hair follicle is infected it is referred to as an external stye, and if the Meibomian gland is affected it is termed an internal stye. Styes can often be self-limiting, but may also be treated by removal of the affected hair follicle. A chalazion, on the other hand, is a painless granuloma of the Meibomian gland, and can be distinguished from a stye by the absence of pain.
The eyelids have a rich vascular supply, with blood being supplied through both the anterior and posterior ciliary arteries. The arterial supply is complex, with an intricate anastomosis between the frontal, lacrimal, superior and inferior palpebral arteries, as well as the angular artery. Venous drainage is into the frontal, angular and superior ophthalmic veins. Other than the lacrimal drainage system, innervation of the eyelids is supplied by the ophthalmic division of the trigeminal nerve. The levator palpebrae superioris is innervated by the superior branch of the oculomotor nerve (CN III), while the Superior Tarsal Muscle receives sympathetic innervation from the superior cervical ganglion.
The eyelid is composed of five major layers, from superficial to deep, that work in tandem to protect the eye from the outside world. These layers include: skin and subcutaneous tissue, the orbicularis oculi, tarsal plates, the levator apparatus, and conjunctiva. Below, each of these layers is discussed in detail.
The skin and subcutaneous tissue make up the most superficial layer of the eyelid. This layer is incredibly thin by comparison to the rest of the body and does not contain any subcutaneous fat, meaning that it is easily distended by oedema or blood. Additionally, it is home to the eyelashes, their accompanying ciliary glands of Moll, and sebaceous glands of Zeis.
The orbicularis oculi muscle is made up of three distinct parts: palpebral, lacrimal, and orbital. It originates from the medial orbital margin, the medial palpebral ligament, and the lacrimal bone and inserts into the skin around the margin of the orbit, as well as the superior and inferior tarsal plates. The palpebral part gently closes the eyelids, the lacrimal part is involved in the drainage of tears, and the orbital part tightly closes the eyelids. This muscle is innervated by the facial nerve (cranial nerve VII, temporal and zygomatic branches).
The tarsal plates reside deep to the palpebral region of the orbicularis oculi muscle, and come in two varieties: the superior tarsus (upper eyelid), and the inferior tarsus (lower eyelid). They serve as the scaffolding of the eyelid and are made from dense connective tissue. The superior tarsus also houses the attachment site of the levator palpebrae superioris. Additionally, the tarsal plates contain Meibomian glands (also known as tarsal glands), which are specialized sebaceous glands that secrete an oily substance which slows the evaporation of the eye's tear film and prevents the eyelids from sticking together when closed.
The levator palpebrae superioris and superior tarsal muscles, both of which are exclusive to the upper eyelid, act together to open the eyelid. The levator palpebrae superioris originates from the lesser wing of sphenoid and inserts into the upper eyelid and superior tarsal plate, while the superior tarsal muscle originates from the underside of the levator palpebrae superioris and inserts into the superior tarsal plate. The levator palpebrae superioris is innervated by the superior branch of the oculomotor nerve (cranial nerve III), and the superior tarsal muscle by sympathetic fibres from the superior cervical ganglion.
The palpebral conjunctiva forms the deepest layer of the eyelid. This thin mucous membrane is reflected onto the sclera of the eyeball (bulbar conjunctiva). It is particularly rich in vessels, with the ophthalmic artery providing the lacrimal, medial palpebral, supraorbital, dorsal nasal, and supratrochlear arteries, while the facial artery supplies the angular branch. The superficial temporal artery supplies the transverse facial artery branch. Venous drainage is provided by a rich network around the eyelid, with medial blood flow draining via the medial palpebral vein into the angular and ophthalmic veins, and lateral flow draining into the superficial temporal vein from the lateral palpebral vein.
The eyelid is innervated by several branches of the trigeminal nerve. The ophthalmic nerve (cranial nerve V1) provides sensory innervation to the upper eyelid, with its supraorbital, supratrochlear, infratrochlear, and lacrimal branches. The maxillary nerve (cranial nerve V2) provides innervation to the lower eyelid, with its infraorbital and zygomaticofacial branches. The facial nerve (cranial nerve VII) provides innervation to the orbicularis oculi, the oculomotor nerve (cranial nerve III) provides innervation to the levator palpebrae superioris, and sympathetic fibres from the superior cervical ganglion to the superior tarsal muscle.
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