The human eye

The human eye is the organ that allows for the detection of light and converts this perception into electrical impulses. In humans, the retina is comprised of two types of photo-sensing cells, the rods, which enable the perception of light and dark, and the cones, responsible for the perception of colors.

The human eye has an anterior-posterior diameter of approximately 24.15 millimeters, horizontal and vertical diameters at the equator of approximately 23.48 millimeters, weighs 7.5 grams, and has a volume of 6.5cc.

The eyeball is given this name due to its globe-like shape, which in turn is housed within a bony cavity and protected by the eyelids. It has on its exterior six muscles that are responsible for eye movements, and also three adherent concentric layers with the functions of vision, nutrition and protection. The outer layer is composed of the cornea and sclera and serves for protection. The middle or vascular layer is made up of the iris, the choroid, the choroid or uvea, and the vascular part of the ciliary body. The inner layer is formed by the retina, which is the nervous part.

There is also the aqueous humor, which is a colorless liquid existing between the cornea and the crystalline. The vitreous humor is a gelatinous substance that fills the entire internal space of the eyeball behind the crystalline. All this works to maintain the spherical shape of the eye.

The crystalline lens is a kind of lens that is inside our eyes. It is located behind the pupil and directs the passage of light to the retina. The retina is composed of nerve cells that carry the image through the optic nerve so the brain can interpret it.

It does not matter whether the crystalline lens becomes thinner or thicker, these changes occur to deflect the passage of light rays in the direction of the yellow spot. As objects get closer, the crystalline lens becomes thicker, and for objects at a distance, it becomes thinner. This is called visual accommodation.

The eye also has eyelids, eyebrows, tear glands, eyelashes, and ocular muscles. The function of the eyelashes or lash hairs is to prevent the entrance of dust and excess light. The eyebrows also function to prevent forehead sweat from coming into contact with the eyes.

The conjunctiva is a membrane that internally lines two folds of skin that are the eyelids. They are responsible for protecting the eyes and spreading the liquid we know as tears.

The liquid that we know as tears is produced in the tear glands, and its function is to spread this liquid by the movements of the eyelids, washing and lubricating the eye.

The blind spot is the location where the optic nerve exits the eye. It is so called because there are no sensory receptors in that location, thus providing no response to stimulation.