Refracting Media

=1c. 2. The Refracting Media=
 * The refracting media are three, viz.: Aqueous humor., Vitreous body.and Crystalline lens.

The Aqueous Humor (humor aqueus)

 * The aqueous humor fills the anterior and posterior chambers of the eyeball.
 * The aqueous humor is small in quantity, has an alkaline reaction, and consists mainly of water, less than one-fiftieth of its weight being solid matter, chiefly chloride of sodium.

The Vitreous Body (corpus vitreum)
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 * The vitreous body forms about four-fifths of the bulb of the eye.
 * The vitreous body fills the concavity of the retina, and is hollowed in front, forming a deep concavity, the hyaloid fossa, for the reception of the lens.
 * The vitreous body is transparent, of the consistence of thin jelly.
 * The vitreous body is composed of an albuminous fluid enclosed in the hyaloid membrane.
 * the hyaloid membrane is a delicate transparent membrane.
 * The vitreous body has been supposed, by Hannover, that from its surface numerous thin lamellæ are prolonged inward in a radiating manner, forming spaces in which the fluid is contained.
 * In the adult, these lamellæ cannot be detected even after careful microscopic examination in the fresh state, but in preparations hardened in weak chromic acid it is possible to make out a distinct lamellation at the periphery of the body.
 * the hyaloid canal is filled with lymph and lined by a prolongation of the hyaloid membrane.
 * the hyaloid canal is a canal in the center of the vitreous body, running from the entrance of the optic nerve to the posterior surface of the lens.
 * the hyaloid canal in the embryonic vitreous body, conveyed the arteria hyaloidea from the central artery of the retina to the back of the lens.
 * The fluid from the vitreous body is nearly pure water with some salts, and a little albumin.
 * The hyaloid membrane envelopes the vitreous body.
 * The hyaloid membrane in front of the ora serrata is thickened by the accession of radial fibers.
 * The The hyaloid membrane in front of the ora serrata is termed the zonula ciliaris of hyaloid membrane
 * the zonula ciliaris of hyaloid membrane is AKA zonule of Zinn.
 * the zonula ciliaris of hyaloid membrane has a series of radially arranged furrows.
 * radially arranged furrows in the the zonula ciliaris of hyaloid membrane accomodate the ciliary processes.
 * the ciliary processes adhere to the zonula ciliaris of hyaloid membrane.
 * the zonula ciliaris of hyaloid membrane splits into two layers.
 * One layer of the zonula ciliaris of hyaloid membrane lines the hyaloid fossa.
 * One layer of the zonula ciliaris of hyaloid membrane is the suspensory ligament of the lens.
 * the suspensory ligament of the lens passes over the ciliary body to be attached to the capsule of the lens a short distance in front of its equator.
 * Scattered and delicate fibers of the suspensory ligament of the lens are also attached to the region of the equator of lens itself.
 * the suspensory ligament of the lens retains the lens in position,
 * the suspensory ligament of the lens is relaxed by the contraction of the meridional fibers of the Ciliaris muscle.
 * the spatia zonularis is AKA canal of Petit.
 * the spatia zonularis is a sacculated canal Behind the suspensory ligament of lens.
 * canal of Petit encircles the equator of the lens;
 * canal of Petit can be easily inflated through a fine blowpipe inserted under the suspensory ligament of lens.
 * No blood vessels penetrate the vitreous body,
 * the vitreous body is nourished by vessels of the retina and vessels of theciliary processes.
 * FIG. 883– The upper half of a sagittal section through the front of the eyeball. (See enlarged image)
 * What is the structure of the lens zonule? The lens zonule suspends the lens and holds it in place. The zonular fibers extend from both the anterior and posterior capsule (arrows 1) of the lens to join the span fibers of the ciliary body and insert on the pars plana and retina.

The Crystalline Lens (lens crystallina)

 * The crystalline lens is enclosed in its capsule.
 * The crystalline lens is situated immediately behind the iris.
 * The crystalline lens is encircled by the ciliary processes, which slightly overlap its margin.
 * The capsule of the lens is AKA capsula lentis.
 * The capsule of the lens is a transparent, structureless membrane.
 * The capsule of the lens closely surrounds the lens.
 * The capsule of the lens is thicker in front than behind.
 * The capsule of the lens is brittle but highly elastic,
 * the edges of The capsule of the lens roll up with the outer surface innermost, when ruptured
 * Behind,The capsule of the lens rests in the hyaloid fossa in the forepart of the vitreous body.
 * In front, The capsule of the lens is in contact with the free border of the iris,
 * but recedes from it at the circumference, thus forming the posterior chamber of the eye;
 * The capsule of the lens is retained in its position chiefly by the suspensory ligament of the lens.
 * The lens is a transparent, biconvex body, the convexity of its anterior being less than that of its posterior surface.
 * The central points of the lens 's surfaces are the anterior pole and posterior poles;
 * a line connecting the anterior pole of lens and the posterior pole of lens constitutes the axis of the lens,
 * while the marginal circumference is termed the equator of the lens.	  8

Structure

 * The lens is made up of soft cortical substance and a firm the nucleus (Fig. 884).
 * radii lentis are Faint lines radiating from the poles to the equator.
 * radii lentis are six or more in adults.
 * radii lentis are only three in the fetus
 * on the anterior surface one line ascends vertically and the other two diverge downward;
 * on the posterior surface one ray descends vertically and the other two diverge upward.
 * They correspond with the free edges of an equal number of septa composed of an amorphous substance, which dip into the substance of the lens. When the lens has been hardened it is seen to consist of a series of concentrically arranged laminæ, each of which is interrupted at the septa referred to. Each lamina is built up of a number of hexagonal, ribbon-like lens fibers, the edges of which are more or less serrated—the serrations fitting between those of neighboring fibers, while the ends of the fibers come into apposition at the septa. The fibers run in a curved manner from the septa on the anterior surface to those on the posterior surface. No fibers pass from pole to pole; they are arranged in such a way that those which begin near the pole on one surface of the lens end near the peripheral extremity of the plane on the other, and vice versa. The fibers of the outer layers of the lens are nucleated, and together form a nuclear layer, most distinct toward the equator. The anterior surface of the lens is covered by a layer of transparent, columnar, nucleated epithelium. At the equator the cells become elongated, and their gradual transition into lens fibers can be traced (Fig. 887).	  9

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 * FIG. 884– The crystalline lens, hardened and divided. (Enlarged.) (See enlarged image)
 * FIG. 885– Diagram to show the direction and arrangement of the radiating lines on the front and back of the fetal lens. A. From the front. B. From the back. (See enlarged image)
 * FIG. 886– Profile views of the lens at different periods of life. 1. In the fetus. 2. In adult life. 3. In old age. (See enlarged image)
 * FIG. 887– Section through the margin of the lens, showing the transition of the epithelium into the lens fibers. (Babuchin.) (See enlarged image)
 * In the fetus, the lens is nearly spherical, and has a slightly reddish tint; it is soft and breaks down readily on the slightest pressure.
 * A small branch from the arteria centralis retinæ runs forward, as already mentioned, through the vitreous body to the posterior part of the capsule of the lens, where its branches radiate and form a plexiform network, which covers the posterior surface of the capsule, and they are continuous around the margin of the capsule with the vessels of the pupillary membrane, and with those of the iris.
 * In the adult, the lens is colorless, transparent, firm in texture, and devoid of vessels. In old age it becomes flattened on both surfaces, slightly opaque, of an amber tint, and increased in density (Fig. 886).	  10

Vessels and Nerves

 * The arteries of the bulb of the eye are the long ciliary artery, the short ciliary artery, and anterior ciliary arteries, and the arteria centralis retinae.
 * The ciliary veins are seen on the outer surface of the choroid,
 * The ciliary veinsare named the venae vorticosae;
 * The ciliary veins converge to four or five equidistant trunks which pierce the sclera midway between the sclero-corneal junction and the porus opticus.
 * Another set of veins accompanies the anterior ciliary arteries.
 * All of these veins open into the ophthalmic veins.
 * The ciliary nerves are derived from the nasociliary nerve and from the ciliary ganglion.	  13