Third eyelid

The third eyelid (or semilunar fold) is a conjunctival fold located in the medial angle of the eye, covering the eyeball when it retracts into the orbit. It is supported by a T-shaped cartilage to which an accessory lacrimal gland is attached.

The movement of the third eyelid is passive; it is the pressure of the eyeball on the orbital fat body that pushes the cartilage outwards. The cartilage reinforces the edge of the third eyelid and is oriented vertically. The base of the T-shaped cartilage is covered by the glandula superficilais of the third eyelid. In pigs, cattle and rabbits, there is also a deep gland, called glandula profunda located a little further into the orbit, which is not directly attached to the cartilage.

The bulbar surface of the third eyelid contains the excretory canaliculi of the glands, as well as agglomerated conjunctival lymphatic nodules.