eyes: (vision)

Eyes: (vision)

Accessory structures of the eye include the eyebrows, eyelashes, eyelids, and lacrimal glands and ducts, all serving to protect the eye

The eyelids, or palpebrae, shade the eyes during sleep, block excessive light, and spread lubrication (blinking). The upper eyelid is far more movable than the lower, moved by the levator palpebrae superioris muscles. The space between the lids, exposing the eye when open, is the palpebral fissure. It's angles are the medial and lateral commissure. The lacrimal caruncle is a small, red elevation in the medical commissure, containing sebaceous and sudoriferous glands. The tarsal plate is a thick fold of connective tissue in the inner eyelid to give it form and structure. The tarsal plates contain Meibomian glands, modified sebaceous glands which secrete an oily substance to keep the eyelids from sticking together. Infection or blockage may result in cysts or tumors called chalazion
The conjunctiva is a thin mucus membrane lining the exposed surface of the eye and the inner surface of the palpebrae. Where it covers the eye is called the bulbar, or ocular, conjunctiva. The conjunctiva covering the inside of the eyelids is the palpebral conjunctiva. Infection or irritation of the conjunctiva results in conjunctivitis
Each lacrimal gland is about the size and shape of an almond. Each is located in the superior lateral orbit. From the glands are about a dozen lacrimal ducts, opening together as the puncta lacrimalia, one small opening in medial commissure. The fluid then drains into two lacrimal canals and into the lacrimal sac in the inferior orbit, leading to the nasolacrimal duct. The nasolacrimal duct transports the lacrimal secretion to the inferior meatus of the nose. The lacrimal secretion contains an anti-bacterial enzyme, lysozyme
At the base of each eyelash are sebaceous ciliary glands, or glands of Zeis, that lubricate the follicle. Infection or blockage forms a sty

The adult eye is about two and one half centimeters in diameter, with only one sixth of it's surface exposed. It can be divided into three layers:

fibrous tunic-- the outermost tunic, white in color. The fibrous tunic can be further divided into the posterior and lateral sclera, and the anterior cornea. The cornea is clear over the aqueous humor, iris and crystalline lens. At the junction of the sclera and cornea is a venous sinus referred to as the canal of Schlemm, where the aqueous humor is drained off. The outer surface of the cornea is covered by the bulbar conjunctiva
vascular tunic-- the middle tunic is the middle layer of the eye, itself composed of three portions. All three together are called the uvea. The choroid is the posterior section of the uvea, a thin, dark brown membrane that lines most of the interior surface of the sclera. Then, around the iris, it becomes the ciliary body. It extends from the ora serrata of the inner tunic (retina) to just behind the sclerocorneal junction. The ora serrata is the ragged termination of the retina. The ciliary body is made up of the ciliary processes, protrusions and folds of the internal surface which secrete the aqueous humor, and the ciliary muscle, a smooth muscle that alters the shape of the lans for near or far vision. The most anterior section of the vascular tunic is the iris. It is made up of circular and radial smooth muscle fibers arranged in the familiar ring-shape. Light passed through the centrally located pupil. The iris is suspended between the cornea and the lens and is attached to the ciliary processes
nervous tunic or retina-- only covers the interior posterior section of the eye, terminating in the ora serrata. The tissue continues at a pigmented, non-seeing section of the retina

The retina contains three areas of neurons: photoreceptor neurons, ganglion neurons, and bipolar neurons. The dendrites of the photoreceptors are called rods and cones for their shapes; the former being longer and more uniform, the second being short, more conical. Rods specialize in seeing in dim light, receiving only black and white. Cones see in brighter light, in color. It is estimated that there are about seven million cones, and fifteen times that is the number of rods in the eye. The cones are mostly concentrated in the central fovea, a small depression in the center of the macula lutea. The macula lutea (yellow spot) is in the precise center of the interior posterior eye. The fovea is the area of sharpest vision. In the central fovea, the are no cones, but the cones increase in density the farther one looks from the fovea

When information passes through the photoreceptors, it moves to the bipolar neurons in the intermediate layer of the nervous tunic to the ganglion neurons. From there, impulses travel to the brain via the optic nerve. Axons of the ganglion neurons exit the eye at the blind spot called the optic disc

Just behind the pupil is the lens, made up of many protein layers, structured like an onion. It is normally totally clear, suspended by suspensitory ligaments. Cataracts are the result of the lens losing it's transparency

The lens divides the hollow cavity of the eye into two. The larger is the posterior cavity, filled with the vitreous humor. The vitreous is jelly-like, helping to maintain pressure to keep the eye from collapsing. It is formed during embryonic life and is not replaced. The anterior cavity is, itself, divided into two cavities. The posterior lies just behind the iris, the anterior in front. This cavity is filled with the aqueous humor. Unlike the vitreous, it is constantly renewed and recycled by the choroid plexuses of the ciliary bodies of the vascular (middle) tunic. Intraocular pressure maintains the shape of the eye. Excessive pressure is referred to as glaucoma. Both humors are used to maintain this pressure and refract light

Impulses travel from the retina to the optic nerve, to the optic chiasma, to the optic tracts, to the lateral geniculate nuclei, to the optic diffusion (of nerves), finally being analyzed in the visual are of the cortex of the occipital lobes (that was a mouth-full!)

Normal (20/20) sight is called emmetropic vision. Nearsightedness, or myopia, occurs when the eye is too long, and the focal point of the image is in front of the retina. Farsightedness, or hypermetropia, is when the eye it too short, focusing behind the retina. The first is corrected by a concave lens, the other, convex. An astigmatism is an irregular lens or cornea. Presbyopia is farsightedness due to the loss of elasticity in the lens

When an image reaches the retina, it is inverted, and must be righted by the brain. The image is also mirror reversed, also corrected in the brain. The eyes converge (convergence) on an object being viewed

The photopigment utilized in rods is rhodopsin, or visual purple, sensitive to low levels of illumination. Rhodopsin is made up of scotopsin (a protein) and retinal (derivative of vitamin A). Retinal can exist in two states, depending on whether or not light is present. Without light, it is cis-retinal, a curved molecule. When light strikes it, cis-retinal becomes a straight molecule, all-trans-retinal. It pulls away from the scotopsin, forming an unstable substance, bathorhodopsin (prelumirhodopsin), and rapidly decomposes as follows: lumirhodopsin, metarhodopsin I, metarhodopsin II, then pararhodopsin. From there, it completely breaks apart into scotopsin and all-trans-retinal. The all-trans-retinal is converted by an enzyme back to cis-retinal, and it is joined to scotopsin, becoming rhodopsin once more

The eye contains three types of cones, whose retinal's are connected to protein fragments, photopsins, rather than scotopsin. The combination works in a similar way to the cycle in the rods, but faster and require brighter light. The three types of cones sense one each red, green and blue light

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