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Diana Mossop

About The Human Eyes

The information on this page is about the human eyes and where applicable other body parts that are associated or related to the eyes.


Cornea

One-sixth of the outer layer of the eye (called "fibrous tunic") bulges forward as the transparent "cornea," which serves as the window of the eye and helps focus entering light rays. It is composed, for the most part, of connective tissue with a thin layer of "epithelium" on the surface. Epithelium is the type of tissue that covers all free body surfaces. The transparency of the cornea is due to the fact that it contains hardly any cells and no blood vessels. On the other hand, it is well supplied with nerve fibers that enter on the margins of the eye and radiate toward the center. These fibers are associated with numerous pain receptors that have a very low threshold. Cold receptors are also abundant in the cornea, but heat and touch receptors seem to be lacking. Along its circumference, the cornea is continuous with "sclera," the white portion of the eye. The sclera makes up the back five-sixths of the outer layer. It provides protection and serves as an attachment for the extrinsic muscles of the eye.

Eye

The eyes are the two organs of sight. They are located in the front upper part of the skull and consist of structures that focus an image onto the retina at the back of the eye which is a network of nerves that convert this image into electrical impulses to be recorded in a region of the brain. The eyeball lies in pads of fat within the orbit, a bony socket that provides protection from injury. Each eyeball is moved by six delicate muscles which are activated and coordinated by nerves in the brain stem. The eyeball has a tough, outer coat called the "sclera," or white part of the eye. The front, circular part is the "cornea" and is transparent. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the "iris" (colored part) with the "pupil" (center). The pupil is black and its diameter is changed by light intensity to control the amount of light which enters the eye. Immediately behind the iris, and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms. The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels which lie just under it called the choroid plexus. The eyeball is sealed off from the outside by a flexible membrane called the "conjunctiva," which is attached to the Skin at the corners of the eye and forms the inner lining of the lids and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness. A "black eye" is really not black at all. A "shiner" is a mixture of purple, red and yellow hues caused by hundreds of tiny, broken blood vessels.

Eye and Rods and Cones

The sensory organs for vision - the eyes - are at the front of the head, but the actual visual sense is provided by areas of the brain at the back and sides. Nerve impulses generated by "rods" and "cones" in the retinas of the eyes travel along the optic nerves to the optic chiasma, where they partially cross over. "Mixed" impulses from both eyes pass through the optic tracts to the striate cortex at the back of the brain and end in the temporal lobe area so that right and left halves of the visual field merge. When light rays reach the retina (the film of the eye's camera), light energy is converted into electrical nerve signals. Crisscrossed with blood vessels, the retina has three layers of microscopically thin nerve cells. Nearest to the lens is a layer of ganglion cells, then a layer of bipolar cells and finally the photoreceptors. It is the photoreceptors that actually process the packets of light energy or photons that impact on the retina, so light must pass through the ganglions and bipolar cells to get to others. There are two types of photoreceptor cells which, because of their shapes, are called "rods" and "cones." Rods are sensitive enough to respond to a single photon, the basic unit of light, but together they create only one coarse, gray image, which is just adequate for seeing in poor light. Fine detail and color come from the cones, but they need a lot more light and work best in broad daylight. Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination of night and day vision. During the 16th and 17th centuries, when witches were said to destroy their victims by the power of the evil eye, hundreds of women were executed just on the evidence that someone had died after receiving an angry look from them. The judges who presided at the trials were so afraid of receiving a curse from these women when passing sentence that some of the defendants were led into the courtroom backwards.

Eye Muscle Control

Each eye is held in place by three pairs of taut, elastic muscles which constantly balance the pull of the others. The superior rectus acts to roll the eyeball back and up, but it is opposed by the inferior rectus. In the same way, the lateral rectus pulls to the side, while the medial rectus pulls toward the nose, and the two oblique muscles roll the eye clockwise or counterclockwise. The muscles of each eye work together to move the eyes in unison. Because of the constant tension in the muscles, they can move the eye very quickly, much faster than any other body movement. The eye muscles work together to carry out no less than seven coordinated movements and allow the eye to track many different kinds of moving object. The first three movements (tremor, drift and flick) are the result of the constant, opposing muscle tension. Tremor causes an almost unseen trembling of a point image (like a spotlight in a dark room), and drift makes the image move slowly off-center. Before the movement becomes really noticeable, there is a quick flick to bring the image to the center. These movements may seem distracting, but they make sure that the image constantly moves over unused parts of the retina and, as a result, the receptors at any spot do not get overloaded with images and effective vision is maintained. Smooth pursuit movements are used to follow objects at a high speed; for example, from word to word and line to line when reading. Binocular vision is created by the separation of the eyes, so that each eye has a slightly different view of the same scene, giving a three dimensional effect. To prevent this from causing double vision, the sixth eye movement, called "vergence," helps out. The eyes turn inward to direct the images directly onto small, rodless areas of the retina. During these movements, the brain registers the amount of tension and uses it to estimate the distance of the object. The complex of the eye movements is the vestibulo-ocular system; it works to keep the image of an object on the rodless areas while the head and body are in motion. This is aided by the vestibular apparatus in the inner ear, which provides the brain with a flow of information about the way that the head is moving. Babies are not able to focus their eyes close up until they are three to six months old, and it may be a year before their eyes can work together all the time, rather than wandering around individually.

Iris of The Eye

The iris is a thin diaphragm composed mostly of connective tissue and smooth muscle fibers that is seen from the outside as the colored portion of the eye. It extends forward from the periphery of the ciliary body and lies between the cornea and the lens. The iris divides the space separating these parts, which is called the "anterior cavity," into an "anterior chamber" (between the cornea and the iris) and a "posterior chamber" (between the iris and the lens).

Optic Disk

In the central region of the retina is a yellowish spot called the "macula lutea". It has a Depression in its center called the "fovea centralis." This depression is the region of the retina that produces the sharpest vision. Toward the center of the fovea centralis is an area called the "optic disk." The nerve fibers leave the eye at this point to become parts of the optic nerve. A central artery and vein also pass through at the optic disk to supply blood to the cells of the inner layer of the eye.

Optic Nerves

The "optic nerves" are the second pair of the cranial nerves, and lead from the eyes to the brain. The sensory cell bodies of the nerve fibers occur in "ganglia" within the eyes. Their axons (conductors of impulses away from their point of origin) pass through the orbits into the brain. The optic nerves hold the number "II" in the order of cranial nerves.

Visual Nerve Pathways

The axons and ganglion cells in the retina leave the eyes to form the "optic nerves." Just in front of the pituitary gland, these nerves form the X-shaped "optic chiasma," and within the chiasma some of the fibers cross over. The fibers from the nasal half of each retina cross over, but those from the temporal sides do not. Specifically, fibers from the nasal half of the left eye and the temporal half of the right eye form the right "optic tract;" and the fibers from the nasal half of the right eye and the temporal half of the left form the left optic tract. The nerve fibers then continue in the optic tracts. Just before they reach the thalamus, a few of them leave to enter nuclei that function in various visual reflexes. Most of the fibers, however, enter the thalamus and form a junction (synapse) in the back of it. From this region the visual impulses enter nerve pathways called "optic radiations," which lead to the visual cortex of the occipital lobes of the brain.

The information on this website is provided for information purposes only and is not intended or recommended as a substitute for professional medical advice. Always seek the advice of your doctor/physician or other qualified health care provider regarding any medical condition or treatment. Some or all of the information on this page may be supplied by a third-party and not controlled by the DianaMossop.com website or authors and is therefore is not the responsibility of the DianaMossop.com website or its authors.

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