Accessory Structures Of The Eye

The eye is a delicate organ which is protected by several structures such as eyebrows, eyelids, eyelashes and extraocular muscles.

Eyebrows:

Eyebrows are two arched ridges of the supraorbital margins of the frontal bone. Numerous hair projects obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.

Eyelids and Eyelashes:

The eyelids are two movable folds of tissue situated above and below the front of each eye. There is short curved hair, the eyelashes situated on their free edges.

The eyelid consists of,

• A thin covering of skin
• Three muscles – the orbicularis oculi, levator palpebrae superioris and
• Muller’s muscles
• A sheet of dense connective tissue, the tatsal plate
• A lining of the conjunctiva

Lacrimal Apparatus:

Lacrimal apparatus consists of,

• Lacrimal gland and its ducts
• Accessory lacrimal glands
• Lacrimal canaliculi
• Lacrimal sac
• Nasolacrimal duct

The tears are secreted by the lacrimal gland and accessory lacrimal glands. They drain into the conjunctival sac by small ducts. The tears then pass into the lacrimal sac (via the two canaliculi), nasolacrimal duct and finally into the nasal cavity (inferior meatus).

The lacrymal glands are present one on the outer upper border of each eye. The lacrymal secretion is watery, alkaline and carries out the following functions

• cleans the eyes
• keeps the eyes moist
• keeps the eyes free of bateria as it contains bacteriolytic lysozyme
• provides nutrition to the cornea

Extraocular Muscles of The Eye:

The eyeballs are moved by six extrinsic muscles, attached at one end to the eyeball and at the other to the walls of the orbital cavity. There are four straight and two oblique muscles.

They consist of striated muscle fibres. Movement of the eyes to look in a particular direction is under voluntary control but co-ordination of movement needed for convergence and accommodation to near or distant vision is under autonomic control.

• The medical rectus rotates the eyeball inwards.
• The lateral rectus rotates the eyeball outwards.
• The superior rectus rotates the eyeball upwards.
• The infertor rectus rotates the eyeball downwards.
• The superior oblique rotates the eyball so that the cornea turns in a downwards and outward directions.
• The inferior oblique rotates the eyeball so that cornea turns ouwards and outwards.

Arterial Supply:

The eye is supplied by the short (about 20 in number) and long ciliary (2 in number) arteries and the central retinal artery. These are branches of the ophthalmic artery, one of the branches of the internal carotid artery.

Venous Drainage:

Venous drainage is done by the short ciliary veins, anterior ciliary veins, 4 vortex veins and the central retinal vein. These eventually empty into the cavernous sinus.

Nerve Supply to The Eye:

The eye is supplied by three types of nerves

• The motor nerves
• The sensory nerves
• The autonomic nerves