The eyelids protect the eye from excessive light or injury and assist in the distribution of tears over the ocular surface. The upper and lower lids meet at the medial and lateral angles (or canthi). The elliptical opening between the eyelids is known as the palpebral aperture or fissure.
From superficial to deep, each eyelid consists of:
The thinnest in the body with minimal subcutaneous tissue devoid of fat. At the lid margin the skin becomes continuous with the conjunctiva. Running along the lid margin, just behind the eyelashes, are multiple tarsal (meibomian) gland orifices. These glands are embedded in the tarsal plates and secrete an oily material which reduces evaporation of tears.
An elliptical striated muscle which is arranged in concentric bands in the eyelids and surrounding tissues. This muscle is supplied by the facial (7th) nerve and is involved in blinking and eyelid closure.
Orbital septum and tarsal plates
The orbital septum is a thin sheet of connective tissue which extends into the eyelids from the orbital margin. Structures anterior to the septum are described as “pre-septal” and the orbital contents are, therefore, “post-septal”. These terms have clinical relevance differentiating pre-septal cellulitis from the much more serious, and potentially sight threatening, orbital cellulitis.
Within the eyelids the orbital septum is significantly thickened to form the tarsal plates which give the eyelids rigidity. The tarsal plates house the tarsal (meibomian) glands and the plate in the upper lid is much larger than the lower lid.
Originates at the orbital apex and travels forward overlying the superior rectus muscle before inserting into the upper eyelid skin and superior border of the tarsal plate. Supplied by the oculomotor (3rd) nerve the levator muscle elevates the upper eyelid.
Beneath the levator muscle, and anterior to the conjunctiva, sits the relatively thin Müllers muscle. Another upper lid elevator this muscle is supplied by the sympathetic system. Over action of Müllers muscle eg thyroid eye disease gives rise to upper lid retraction, whereas under activity eg Horners syndrome results in a mild ptosis.
This is a thin mucous membrane that lines the inside of the eyelids (palpebral conjunctiva) and is reflected at the superior and inferior fornices onto the anterior surface of the globe (bulbar conjunctiva).