The Human Eye

The eyelids have four distinct layers. The skin, muscle, Connective, and Conjunctiva. The skin is the outer surface of the lids. The muscle, Orbicularis Oculi makes the muscles contract which opens and closes the lid. The connective protects the lids, and lastly the conjunctiva lines the inner surfaces and covers the anterior surfaces.

The lacrimal apparatus consists of the lacrimal gland which secretes tears to take away bad thing or when crying. This also consists of the six extrinsic muscles that move the eye.

The three layers that make up the eye are, the fibrous outer tunic, a vascular middle tunic, and a nervous inner tunic

Outer Tunic: This consists of the cornea which helps focus light rays, the sclera, which is the white portion of the eye. The sclera has many fibers. And lastly the optic nerve which connects to something located in the back of the eye.

Middle Tunic:
Choroids coat: Loosely joined to the sclera and nourishes surrounding tissues.
Ciliary body: Thicker part of the middle tunic, which extends forward from the choroids coat and forms and internal ring around the front of the eye.
Lens: Held by the suspensory ligaments, it is a clear, membrane like structure composed of mostly of intercellular material. The lens focuses on far and close objects
Iris: Lies between the cornea and the lens, the Iris is a thin diaphragm.
Aqueous humor: Helps maintain the shape of the eye. It secretes a watery fluid between the lens and the cornea to nourish it.
Pupil: Black part in the middle of the Iris which expands and contracts to control the light

Inner Tunic
Retina: Contains visual receptor cells. The retina is transparent and it lies at the margin of the ciliary body.
Fovea Centralis: Yellow part that produces the sharpest image.
Optic disk: Supply blood to the cells of the inner tunic. Nerve fibers to the retina connect to the optic disk.
Vitreous Humor: This is a gel like substance that helps maintain the shape of the eye.

The lens adjusts to shape with the help of Ciliary Muscles fibers to focus. This is done by moving the choroids coat and adjusting how much concave and convex it is.

The Iris is a muscular diaphragm that controls the amount of light that enters the eye.

The aqueous humor is a watery fluid that circulates through the pupil. The vitreous humor is a gel like substance and helps keep the shape of the eye.

The fovea centralis is the part of the sharpest vision where as the optic disk is the region in the retina where nerve fibers exit to become part of the optic nerve.

When a person sees something, either the object is jumping off something, or light waves are reflected at it. These light eaves enter the eyes and an image of the object is focused on the retina through refractions.

Rods have long, thing projections at their ends and are responsible for vision in dimmer light because they are so sensitive. Cones have short blunt progressions and provide colored visions. There are three different colored cones in all that make all of the colors in the world.

Rods provide lens precise images because nerve fibers from many rods converge, their impulses transmitted to the brain on the same nerve fiber. This convergence is less common in cones, therefore the cones bring more acute vision

The light sensitive bio-chemical in rods is called rhodopsin. Decomposition of rhodopsin molecules activates an enzyme that initiates a series of reactions altering the permeability of the rod cell membrane. As a result, a complex pattern of nerve impulses originates in the retina. The impulses travel away from the retina along the optic nerve into the brain, where they are interpreted as vision.

The cones of the eye are made up of three basic colors. These colors are red, green, and blue. The highest wavelength of these colors is red and the shortest wavelength of these colors is blue. The cones are specifically trained to determine the color by the wavelengths. For example the grass is green because it is of medium wavelengths. Since our eyes are trained to see that wavelength as green, our brain always sees green.

The nerve impulse first goes to the optic nerve then the optic chiasma, next it goes through the optic tract. Just before the thalamus a few of them enter nuclei, next the impulse goes to the thalamus, and then the Optic radiations and last it reaches the visual cortex of the occipital lobe

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