The amount of light entering the human eye is regulated by the diameter of the pupil. The pupil is the dark spot at the center of a healthy iris that, according to the National Institutes of Health, rests immediately in front of the eye's lens.Know More
The pupil plays much the same role in the vertebrate eye as a shutter does in a camera. When the pupil constricts, the amount of light that is able to pass through to the lens is reduced. An open or dilated pupil is able to admit more light and appears as a larger dark circle in the center of the iris than does the pinpoint of a constricted pupil. This expansion and contraction is controlled by tiny muscles in the iris, which is the colored part of the eye. About.com lists normal pupil size in humans as ranging between 2 mm and 5 mm, depending on external light conditions, with young people typically having somewhat larger pupils than adults.
The appearance and behavior of the pupil can be useful to doctors, according to About.com. Unequal pupil diameter naturally occurs among some people, but unequal or abnormal pupils can be a sign of head trauma, cancer and certain diseases such as syphilis.Learn more in Organs
The two main defects of the human eye are myopia, which is nearsightedness, and hyperopia, which is farsightedness; both myopia and hyperopia can be treated by wearing glasses. Another major defect is when the curvature of the cornea is irregular: this condition is called astigmatism. Astigmatism can be treated by wearing glasses or undergoing surgery.Full Answer >
The small intestine is approximately 20 feet in length and 1 inch in diameter. The function of the small intestine is to absorb nutrients from food and drink.Full Answer >
The human eye sees light with the pupil and the retina. Located inside the retina are two types of special cells called photoreceptor cells. There are two varieties of photoreceptor cells known as cones and rods.Full Answer >
According to the Eye Care Office of Ofner, Neale and Fleming, cataracts of the eye create a clouding of the lens inside the eye, behind the pupil or iris (the colored portion of the eye), that is often mistaken for a film over the eye. Cataracts are a common aspect to the aging process. Early signs of cataracts include decreased distance vision and decreased vision while reading.Full Answer >