Structure of the Human Eye - Practice Questions & MCQ

• Our paired eyes are located in sockets of the skull called orbits.
• The adult human eyeball is nearly a spherical structure. 
• The eyeball is an elongated sphere about 2.5 cm in diameter.
• The wall of the eyeball is composed of three layers:

        1. External Layer or Fibrous Coat: 

• Sclera forms ⅔ rd of the outer layer and Cornea ⅓ rd 
• Sclera is tough and made up of dense connective tissue
• Cornea is transparent and dome shaped. It collects the light rays and acts as refracting surface
• The junction of sclera and cornea has canal of schlemm

       2. Middle Layer or Vascular Coat or Uvea

• The middle layer, choroid, contains many blood vessels and looks bluish in colour. 
• The choroid layer is thin over the posterior two-thirds of the eyeball, but it becomes thick in the anterior part to form the ciliary body. 
• The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
• The eyeball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. 
• In front of the lens, the aperture surrounded by the iris is called the pupil. The diameter of the pupil is regulated by the muscle fibres of iris.

       3.The inner layer or Neural Coat:

• The inner layer is the retina and it contains three layers of neural cells from inside to outside – ganglion cells, bipolar cells and photoreceptor cells.
• The pigmented layer lies close to the choroid.
• The point where the retina attaches to the choroid is the Ora Serrata 

Lens:

• It is a transparent, biconvex and elastic structure.
• It bends the light waves as they pass through it.
• It divides the cavity of eye into two parts : Anterior and Posterior
• The action of the ciliary muscles can cause changes in the focal length of the lens.

Aqueous Humor:

• This is the Anterior cavity
• This chamber lies between the cornea and lens is called the aqueous chamber.
• It is filled with a watery fluid called the aqueous humour.
• It is secreted by the ciliary processes.
• It provides nourishment to the lens and cornea.
• It is continuously drained out of the eyes.

Vitreous Humor:

• This is the posterior cavity
• This is a bigger space and lies between the lens and retina and it is called the vitreous chamber.
• It is filled with vitreous fluid.
• It is not drained like the aqueous humour.
• This fluid helps to maintain the intraoccular pressure and also the shape of the eyeball