Question

Describe in detail the part of the eye that controls pupil diameter. Also, describe the cornea and exactly what does it cover? What is the conjunctiva and exactly what does it cover? Additionally, what is the structure that anchors the extrinsic muscles to the eye? Additionally, trace the path of light through the eye to the retina.

Answer 1

Pupil size influences accommodation by controlling the amount of light entering the eye. In a bright light the pupils are constricted. In a dim light they are dilated.
If the pupils were dilated in a bright light, too much light would enter the eye and damage the retina. In a dim light, if the pupils were constricted, insufficient light would enter the eye to activate the photosensitive pigments in the rods and cones which stimulate the nerve endings in the retina.
The iris consists of one layer of circular and one of radiating smooth muscle fibres. Contraction of the circular fibres constricts the pupil, and contraction of the radiating fibres dilates it. The size of the pupil is controlled by the nerves of the autonomic nervous system. Sympathetic stimulation dilates the pupil and parasympathetic stimulation causes contraction of the pupil.

Cornea is a clear transparent and elliptical structure with a smooth shining surface, avascular, watch-glass like structure. It forms anterior one-sixth of the outer fibrous coat of the eyeball. It covers anterior chamber , iris and pupil.

Conjunctiva is a translucent mucous membrane which lines the posterior surface of the eyelids and anterior aspect of the eyeball. The name conjunctiva (conjoin: to join) has been given to this mucous membrane owing to the fact that it joins the eyeball to the lids. It stretches from the lid margin to the limbus, and encloses a complex space called conjunctival sac which is open in front at the palpebral fissure. It covers sclera.

Each eyeball is suspended by extraocular muscles and fascial sheaths in a quadrilateral pyramid- shaped bony cavity called orbit . Each eyeball is located in the anterior orbit, nearer to the roof and lateral wall than to the floor and medial wall. Each eye is protected anteriorly by two shutters called the eyelids. The anterior part of the sclera and posterior surface of lids are lined by a thin membrane called conjunctiva. For smooth functioning, the cornea and conjunctiva are to be kept moist by tears which are produced by lacrimal gland and drained by the lacrimal passages. These structures (eyelids, eyebrows, conjunctiva and lacrimal apparatus) are collectively called ‘the appendages of the eye’.

Light passes through the cornea,lens and retina.