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You are watching: Tough, white connective tissue forms this portion of the fibrous layer of the eye:

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. Second edition. Sunderland (MA): Sinauer Associates; 2001.


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The eye is a fluid-filled ball enclosed by 3 layers of organization (Figure 11.1). Many of the outer layer is composed of a hard white fibrous tissue, the sclera. In ~ the front of the eye, however, this opaque outer layer is transformed right into the cornea, a devoted transparent organization that permits irradiate rays to enter the eye. The middle layer the tissue has three unique but constant structures: the iris, the ciliary body, and also the choroid. The iris is the colored part of the eye that can be seen through the cornea. It contains two to adjust of muscles with opposing actions, which allow the size of the pupil (the opened in that center) come be readjusted under neural control. The ciliary body is a ring of tissue that encircles the lens and also includes a muscular component the is essential for adjusting the refractive power of the lens, and a vascular component (the so-called ciliary processes) the produces the liquid that filling the prior of the eye. The choroid is composed of a rich capillary bed that serves as the main source of blood supply for the photoreceptors the the retina. Only the innermost great of the eye, the retina, consists of neurons that room sensitive to light and are qualified of transmitting intuitive signals to central targets.


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En path to the retina, irradiate passes with the cornea, the lens, and two unique fluid environments. The anterior chamber, the room between the lens and the cornea, is filled through aqueous humor, a clear, watery liquid that supplies nutrients to these structures and to the lens. Aqueous humor is created by the ciliary procedures in the posterior chamber (the region between the lens and the iris) and also flows right into the anterior chamber v the pupil. A devoted meshwork of cells the lies at the junction that the iris and the cornea is responsible for its uptake. Under common conditions, the prices of aqueous feeling production and uptake room in equilibrium, ensuring a continuous intraocular pressure. Abnormally high levels of intraocular pressure, which happen in glaucoma, can reduce the blood supply to the eye and eventually damages retinal neurons.

The space between the ago of the lens and the surface ar of the retina is filled v a thick, gelatinous substance called the vitreous humor, which accounts for about 80% the the volume that the eye. In enhancement to preserving the shape of the eye, the vitreous humor includes phagocytic cell that eliminate blood and also other debris that can otherwise interfere with light transmission. The housekeeping ability of the vitreous humor space limited, however, as a large number that middle-aged and also elderly people with vitreal “floaters” will attest. Floaters room collections that debris too huge for phagocytic intake that as such remain to actors annoying shadows top top the retina; they generally arise when the aging vitreous membrane pulls far from the overly lengthy eyeball of myopic people (Box A).

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