3D-HuBEd No.8 -EYES-

The eye is an organ for looking at an object. Two eyeballs fall into the hollows of a skull, and are protected from the outside by the eyelid and tears. The eyeballs are connected directly with the brain by the optic nerves. Six muscles sticked on an eyeball put an eyeball into action. A ray of light focuses into an image on the retina by the work of the cornea and the lens. Countless visual cells referred as a rod and a cone stand in a row in the retina, and change an image to a pattern of stimuli. These stimuli are transmitted to the brain via the optic nerves. In the brain reached information is processed and changed into a definite image.

Refer to chapter 0 for more information on how to use this textbook.

　　8.1 The structure of the eye
　　8.2 The accommodation
　　8.3 The adaptation to luminosity

The structure of the eye

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The whole structure looks very much like a camera. The cornea's roles are protecting the lens and refracting the ray of light as a colorless filter. A lens is like a camera's lens. It has elasticity, and changes its thickness with that the muscle of ciliary body around it expands or contracts. It is used for the accommodation. The iris comes under a camera's stop, and has a role of the adaptation to luminosity that accommodates volume of light coming into the eyeball. A vitreous humor is a jelly substance, and keeps shape of an eye. An aqueous humor gives the lens and the cornea nutrition. The retina looks like a film, and changes reflected an image into electric signals. The visual cell in the retina is classified under a rod working in the dark but not sensing color and a cone sensing color in the light. A visual cell makes connection with nerve fiber. A bundle of about a million nerve fibers is called optic nerves. Sight arises from that optic nerves signal the visual cortex of the brain. An eyeball has next three membranes.

sclera
choroid
retina

8.2 The accommodation

In optically a normal eye, without accommodation an image of an object in the infinite distance is reflected in the retina. As an object comes near, the position of the image shifts at the back of the surface of the retina, so the retina cannot get a clear image. When distance from a subject to a camera changes, we bring into focus by changing distance between the lens and the film. An eye of human(mammal) accommodates the lens by not changing its position but changing its curvature and its refraction. This function of accommodation to fucus into a clear image on the retina wether an object is near or far is referred as the accommodation.

Without accommodation the lens is flat, because it is given tension to the outside by the zonular ligament stretched between its membrane and the ciliary body. To accommodate for a near object, the ciliary muscle is constricted. When the zonular ligament is slackened, the lens tries to expand into a sphere by its elasticity. In this way the accommodation is done by changing tension of the lens. These ciliary muscles are controlled by the parasympathetic nerve.

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8.3 The adaptation to luminosity

Following easy processes make observation of the adaptation to luminosity possible.

In the dark, expose the face of a testee to light of the floodlight.
Cut off the light coming into the eye by a celluloid sheet.

At the time, the pupil dilates very fast. To do away a celluloid sheet, the pupil contracts as before. When the light reaching the retina reduces, by constriction of radiating muscle in the iris(ciliary body) the retina contracts. In this way, the iris eases quickly value of the light coming into the eye. Such function of the eye is referred as the adaptation to luminosity generally. This function is homeostasis realized by that the effectors around the eye receive the signals from the brain.

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