HUMAN EYE:

Sight is an amazing process made possible by many parts of the eye working together. Light enters the eye and is bent or refracted by the cornea (the window of the eye) through the pupil (the opening in the iris). This light passes through the lens (located behind the pupil). This completes refraction by fine tuning the focused light onto the retina. The retina changes the light (energy) into electric impulses that are carried through the optic nerve to the vision center (occipital cortex) of the brain where the image is interpreted. A summary of the eye's structures and their functions follows:

THE CORNEA:

The cornea is the "window" of the eye (like a watch crystal). It is the clear part of the eye, through which the colored part of the eye is seen. It is the main source of refraction. The cornea is made up of five layers of strong clear tissue. The first layer (epithelium) is made up of rapidly-replaced cells that allow for fast healing (24-48 hrs) of surface injuries. The last four layers add rigidity, provide a barrier against infection and keep the cornea clear.

THE SCLERA:

The outer "white part" of an eye is the sclera. This tough structure is the outer wall of the eye that gives protection to the delicate inner structures.

THE CHOROID:

This structure, between the sclera and the retina, is made up of blood vessels that provide nourishment to the eye.

THE IRIS:

This colored part of the eye has very fine muscles to control the size of the pupil.

THE PUPIL:

The pupil is the black-appearing spot in the center of the iris. Its size changes since its function is to control the amount of light reaching the retina. In the dark, it expands allowing more light to enter. It contracts in bright light to keep out excess light.

THE LENS:

This controls 1/3 of the refraction of light that enters the eye (the cornea, the other 2/3). Located just behind the pupil it allows for changing of focus from distance to near objects by altering its shape. This changing focus is called accommodation. As a person ages the lens hardens and accommodation becomes more difficult.

THE ZONULES:

These "threads" attach the lens to the ciliary muscle and help the lens to change its curvature during accomodation.

THE CILIARY BODY

This contains two main structures. The first is a muscle that contracts and expands to control the curvature of the lens during accommodation. The second is a gland that secretes aqueous humor.


THE AQUEOUS HUMOUR:
This fluid is produced by the ciliary body and circulates in the front part of the eye. It provides nourishment to the front parts of the eye and maintains the eye pressure.


THE RETINA:
This membrane lines the inside wall of the eye. It contains photoreceptors (rods and cones) that change light into sight by converting light into electrical impulses. These electrical messages are sent from the retina to the brain and interpreted as images.

THE MACULA:
This tiny part of the retina is the central focusing spott. It is responsible for seeing details (such as reading) and also for color vision.

THE OPTIC NERVE:

This nerve is the pathway that the light rays take from the retina to the processing center of the brain. It actually is made of about a million tiny nerves bundled together.

THE OPTIC DISC
Tthis area is not sensitive to light and it is often referred to as the "blind spot". It is where the retina meets the optic nerve.

THE VIRTEROUS GEL:
This clear gel fills the central core of the eye. It helps to maintain a spherical shape to the eye.

--THE SKIN:

From the biological point of view, skin can be described as a layered structure,Skin is the largest organ in our body and is essential for survival. Functionally, it maintains a rich and reactive blood supply that provides our body essential nutrients while controlling body temperature. Externally, skin often defines our emotional wellbeing through appearance.

LAYERS OF SKIN:
Our skin is made up of two layers - the epidermis outer layer and the underlying dermis. The epidermis protects against infection and dehydration. The dermis is responsible for the elasticity of our skin, which is vital to unimpaired body movement.

EPIDERMIS:
The epidermis is the outermost layer of skin. While only 0.1 mm thick, it has five layers, but contains no blood vessels. Your body reproduces a new epidermis layer approximately every 30 days. When damaged, the epidermis is capable of healing itself, usually without scarring.

DERMIS:
The dermis is the sensitive, vascular, inner layer of skin below the epidermis. It supports the epidermis with a necessary blood supply through a network of connective tissues. The dermis provides your skin with strength and elasticity. When injured or damaged, the dermis does not regenerate. When completely destroyed, our skin may heal itself by scar formation.

SUBCUTANEOUS TISSUE:
Subcutaneous tissue or fat lies beneath the dermis and protects bone, tendon, muscle and other tissues.