The upper and lower eyelids work together to function as a sphincter, the protectors of the eye, and from the anatomical point of view must be considered as one functioning unit, just like the upper and lower lips collectively form the sphincter of the mouth.
Form and substance are given to the lids by what might be considered their skeleton, for it is made of the same substance as bone, i.e. collagen, but it is not mineralized and its fibers are arranged in a linear manner. The structure is known collectively as the tarsal plate composed of upper and lower tarsus (plural tarsi). The name is derived from the Greek implying, “flat,” used for the wing of a bird, which conceivably was thought to flutter like the eyelid. If one squeezes the upper eyelid between finger and thumb the tarsal plate is readily discerned beneath the skin.
The semi-oval tarsus of the upper eyelid (superior tarsus) is 10mms in height in its center, tapering to 2mms at each end, with a thickened straight lower border. The tarsus of the lower eyelid (inferior tarsus) is about half the height of the upper, but otherwise of the same general shape. An orbital septum is a thin layer of fascia, continuous with the periosteum of the bony orbit, it fills the space between the orbit and upper tarsus, to which it is attached.
At what might be thought of as the corners of the eye, the united upper and lower tarsi are stabilized to the orbit by “ligaments,” bands of organized collagen fibers; the lateral palpebral ligament inserts into the zygomatic bone, the medial palpebral ligament is attached to the maxillary process at the medial orbital margin.
The broad tendon of levator palpabrae superioris muscle, which “opens” the eye, blends into the fibers of the upper third of the superior tarsus.