The Senses

The Senses

* The human being is able to communicate with the surrounding environment because of a miraculous network of nerves coordinated with the organs of the five special senses, which allow us to see, hear, taste, smell, and touch.

* For additional protection, the outside of the eye is covered with tough, white fibrous tissue called the sclera.

* Under the sclera is another covering called the choroid, which contains the blood vessels that serve the tissues of the eye.

* Bothe the eye and the camera have a lens to focus and image onto a surface for "recording." In the camera this surface is the film or the sensors in a digital camera. In the eye it is the retina.

* In the camera the distance between the film and the camera lens is adjusted to bring the picture into focus before it is recorded on the film. In the eye, the shape of the elastic lens is automatically altered by ciliary muscles of the ciliary body to focus objects onto the retina. When the ciliary body contracts, the lens becomes rounder in a process known as accommodation to permit near vision.

* The aperture of the camera is similar to the iris of the eye; the size of their openings is adjusted to allow varying amounts of light to enter.

* When the light is bright, the circular muscle fibers of the iris contract, reducing the size of the pupil, thereby permitting less light to enter.

* If the cornea develops an abnormal shape, vision becomes blurred and the result may be a disorder known as astigmatism.

* A mucous membrane called the conjunctiva lines the inner surfaces of the eyelids and covers the anterior sclera surface of the eye.

* Aqueous humor-a watery, transparent liquid that circulates between the anterior and posterior chambers of the eye.

* Amblyopia-a condition known as lazy eye because it causes the turning eye to become "lazy." It is most prevalent in children under the age of 5.

* Arcus Senilis- The condition accompanies normal aging and is included in this discussion because it is so prevalent. It results in a thin grayish-white arc or circle not quite at the edge of the cornea. If it is present in young people, it may suggest hypercholesterolemia (high level of cholesterol in the blood.)

* Blepharitis (Lid Margin Disease)- This is a persistent inflammation of the edges of the eyelids involving the hair follicles and glands. The condition is usually associated with seborrhea of the scalp (dandruff), oily skin, or dry eyes.

* Cataract-This gradually developing opacity (cloudiness) of the lens occurs most frequently in persons over 70 years of age as part of the aging process.

* Conjunctivitis-This condition is caused by inflammation of the conjunctiva. It usually begins in one eye, spreading rapidly to the other from contamination by a wash cloth or by the hands. Because it is highly contagious, other family members should not share towels, wash cloths, or pillows with the infected persons.

* Corneal Abrasion- A scratch or trauma to the cornea, usually from a foreign body in the eye. Even if the eye waters profusely to cleanse the surface, the scratch (abrasion) remains. Vision may be affected if the location and extent of injury are significant.

* Diabetic Retinopathy-Symptoms result from an edematous retina, which causes light to scatter. Tiny capillary walls thicken and show evidence of dilation, twisting, and hemorrhage. This causes glare, blurred vision, and reduced visual acuity. If diagnosed and treated early, prognosis is good for simple forms; in extensive forms, prognosis is poor, with 50% becoming blind within 5 years.

* Glaucoma-This condition of excessive intraocular pressure results in atrophy (wasting away) of the optic nerve. Glaucoma causes severe visual impairment and eventually blindness. It occurs in 2% of adults over age 40 and accounts for 15% of all blindness in the United States. It is the most easily prevented cause of blindness.

* There are two main forms of glaucoma: open-angle (most common) and closed-angle. Each form has its own parameters. Glaucoma causes severe visual defects because the increased intraocular pressure within the eyeball causes pressure against the blood vessels of the retina, reducing the blood supply and destroying retinal nerve fibers. Glaucoma is diagnosed with evidence of increased intraocular pressure as measured by a tonometer. The pupil of the eye is then dilated in order to examine the optic disc. Millions of nerve fibers go from the retina to the optic nerve. They meet at the optic disk, the "blind spot" at the back of the retina. As the nerve fibers are destroyed, the disc begins to change and appears to hollow out. This is referred to as "cupping." As more fibers die, the cup becomes deeper, and more vision is lost.

* The fluid that exerts pressure within the eye can result from either an overproduction of the aqueous humor produced by the epithelium of the ciliary body or the obstruction of its outflow circulating mechanisms to the canal of Schlemm for absorption into venous circulation.

* Myopia-This condition is a defect in vision that is also known as nearsightedness. Objects can be seen distinctly only when close to the eyes. The rate of incidence is believed to be around 11 million Americans.

* Presbyopia-This condition is characterized by inability of the lens to accommodate for near vision. Presbyopia occurs as part of the normal aging process.

* Ptosis-The drooping of the upper eyelid.

* Strabismus-a condition in which one eye deviates with the gaze being abnormally inward or outward, higher or lower than the other eye. An abnormally inward gaze (convergent or "cross-eye") is also called esotropia, and an abnormally outward gaze (divergent or "walleye") is also known as extropia.

* The ear is capable of receiving vibrations in the air and translating them into the sounds we recognize: the more vibrations per second, the higher the frequency, or pitch, of the sound; the stronger the vibration, the louder the sound.

* The outer ear-Vibrations are picked up by the pinna (auricle)