Eyelids

Eyelids are two folds that can be closed by a group of muscles to form a protecting covering over the eyeball against excessive light and injury.

Function
Their capital functions is the regilar spread of tears and other secretions on the eye surface to keep it moist. They keep the eyes from drying out when asleep. Normally the eyelids close every six seconds by reflex action. If dust or other allergens appear in the eye, eyelids close more often and more tears are produces. The lipid secretion of the Meibomia glands lubricates the eyelids prevents evaporation of the eye's tear film.

Anatomy
The 4 layers of the eyelid are:

Skin
This is a very thin fold of skin that contains ciliary glands ( sebaceous glands0 and the cilia ( eyelashes) that protect the eye from the dust and debris.

Muscular layer
This is represented of:
 *  Superior Eyelid:
 * Orbicularis oculi muscle (palpebral part)
 * Levator palpebral superioris
 * Superior tarsal muscle


 * Inferior Eyelid:
 * Orbicularis oculi muscle (palpebral part)
 * Inferior tarsal muscle

Fibrous layer
This is represented by the tarsal plates which is the skeleton of the eyelids.Tarsal plates or palpebral cartilages are two thick plates of connective tissue that give support to eyelids. Are attached to the margin of the orbit by the palpebral ligaments ( medial and lateral) and contain tarsal glands and tarsal muscle. The tarsal glands named also Meibomian glands are 30-40 in upperlid and 20-30 in lower lid. These glands are a special type of sebaceous gland that supply the "meibum" a lipid secretion that prevents evaporation of the eye's tear film. Dysfunction of these glands may cause"dry eye" or blepharitis.

Inner layer
This consists of the conjunctiva, a mucous membrane that is attached to the eye ball to the eyelids (palpebral conjunctiva) and to the orbit ( bulbar conjunctiva)

Occulomotor (cranial nerve III)
Occulomotor with its superior branch innervates the Levator palpebral superior which is resposible for lifting of the superior eyelid.

Trigeminal nerve (cranial nerve V)

 * V1 Ophthalmic nerve. This is the first branch of Trigeminal nerve and gives three branches:
 * a.Lacrimal nerve. Innervates the skin of the lateral corner of the eye and via parasympathetic fibers from the zygomatic nerve the lacrimal gland
 * b.Frontal nerve. Gives the Supraorbital nerve which innervate the conjunctiva and the upper eyelid and the Supratrochlear nerve which innervates the medial corner of the eye.
 * c.Nasociliary nerve. Gives the Infratrochlear nerve which innervates the medial corner of the eye and the lower eyelid


 * V2 Maxillary nerve. This is the second branch of the Trigeminal nerve and gives the Infraorbital nerve that innervates the lower eyelid.

Facial nerve (cranial nerve VII)
The facial nerve innervates the Orbicularis oculi muscle which is responsible for shutting the eyelids.

The Sympathetic nervous system
This is responsible for the innervation of the smooth tarsal muscle superior and inferior which control the size of the palpebral fissure.