The eye

Th Eyeball

The Eye(ball) is placed into the orbit, which protects, supports, and maximizes the function of the eye. To move an eye by rotating about its vertical, horizontal, and antero-posterior axes, six muscles (medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique) are present.

A. The cornea

AugeThe cornea is the front part of the eye. It is separated from the atmosphere by a film of tears, on which the contact lens rests. Its appearance can be compared to the lens of a camera, a spherical form that flattens towards the rims.The form and the regularity of the surface of the cornea are important factors in the choice of the geometry of a contact lens (diameter, radius).

B. The retina

The retina is a light sensitive part inside the inner layer of the eye. It consists of different types of photoreceptor cells that receive light and transform it into image-forming signals. These signals are transmitted through the optic nerve to the brain. In this respect, the retina is comparable to the film in a camera.

C. The lens

The lens is, comparable to a contact lens, a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens has, activated by the eye-muscle, the ability to change its surface curvature. By altering the shape the focal distance of the eye is changed. It can thus focus on objects at various distances, allowing a sharp image of the object of interest to be formed on the retina. This adjustment of the lens is known as accommodation. It is similar to the focusing of a photographic camera via movement of its lenses. Unfortunately, the ability for accommodation decreases with age and results to the inability of the eye to focus on nearby objects. This phenomenon is called Presbyopia.

Sclera

The sclera is a dense fibrous opaque white outer coat enclosing the eyeball, except the part covered by the cornea

Vitreous

The vitreous cavity is located behind the lens and in front of the retina. It is filled with a gel-like fluid, called the vitreous humor. The vitreous places 2/3 part of the eye and helps to maintain the shape of the eye.

Iris and Pupil

The opaque muscular contractile diaphragm possesses a deeply pigmented posterior surface, which excludes the passage of light except through the pupil, and a coloured anterior surface which determines the colour of the eye. A lot of pigment concentration shows a dark brown iris colour, a lack of pigments a bright blue iris. The iris controls the amount of light that enters the eye with a ring of muscle fibres around the pupil, which, when they contract, causes the pupil to constrict and become smaller. A other set of radiate muscle fibres outward from the pupil. When these muscles contract, the pupil dilates and becomes larger.

Retina, receptors and macula

The Retina is a layer of nervous tissue, covering the back two-thirds of the eyeball. There occurs the stimulation by light and initiates an electrochemical reaction in which electrical impulses are transmitted to the brain, producing the sensation of vision. The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight. In the retina there are 2 different receptors; “rods” ar light-sensitive receptor cells of the retina that function in dim-light and night vision, There are about 120 to 130 million rods per eye, non of them are in the fovea. The “cones” are light-sensitive receptor cells of the retina that function in color vision and in the perception of fine detail. The most of the 6½ to 7 million cones are in the macula region, specially in the fovea. The macula area, placed in the middle of the retina, providing the clearest, most distinct vision. In the center of the macula, an area of 1.5 mm in diamter is the fovea. The fovea is the part where all of the photoreceptors are cones; there are no rods in the fovea. The fovea is the point of sharpest, most acute visual acuity.

Tear glands

The tear glands located near the eye and in the eyelids which produce the lipid, lacrimal, and mucoid layers of the tear film coating the anterior surface of the cornea.

You Tube videos

about the eye