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Parts Of Human Eye And Their Function

Edward S. Perkins Emeritus Professor of Ophthalmology, University of Iowa, Iowa City. Co-author of Atlas of Diseases of the Eye.

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Hugh Davson Honorary Research Fellow and Fellow, Department of Physiology, University College, University of London. Author of Physiology of the Eye; editor of The Eye.

The Editorial Board of Encyclopaedia Encyclopaedia’s editors oversee subject areas in which they have extensive knowledge, either from years of experience gained working on that content or through study for an advanced degree. They write new content and verify and edit content received from contributors.

Human eye, in humans, specialized sense capable of receiving visual images, which are then carried to the brain.

The eye is protected from mechanical injury by being enclosed in a cavity or orbit, which consists of portions of several of the bones of the skull to form a square pyramid, the apex of which points back towards the head. The floor of the orbit thus consists of parts of the maxilla, zygomatic and palatine bones, while the roof consists of the orbital plate of the front bone and, behind it, by the lower wing of the sphenoid. The optic foramen, the opening through which the optic nerve runs back into the brain and the great ophthalmic artery enters the orbit, is on the nasal side of the apex; the superior orbital fissure is a larger hole through which large arteries and nerves pass. These nerves can carry nonvisual sensory messages—eg. pain—or it could be motor nerves that control the muscles of the eye. There are other slits and channels that transmit nerves and blood vessels. The eyeball and its functional muscles are surrounded by a layer of orbital fat that acts much like a cushion, allowing a smooth rotation of the eyeball around a virtually fixed point, the center of rotation. The protrusion of the eyeballs—proptosis—in exophthalmic goitre is caused by the collection of fluid in the orbital fat tissue.

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It is extremely important that the front surface of the eyeball, the cornea, remains moist. This is achieved by the eyelids, which during waking hours sweep the secretions of the lacrimal apparatus and other glands over the surface at regular intervals, and which during sleep cover the eyes and prevent evaporation. The lids have the additional function of preventing injury from foreign bodies, through the action of the blink reflex. The eyelids are essentially folds of tissue that cover the front of the orbit and, when the eye is open, leave an almond-shaped opening. The ends of the almond are called canthi; the one closest to the nose is the inner, and the other is the outer canthus. The lid can be divided into four layers: (1) the skin, which contains glands that open onto the surface of the eyelid margin, and the eyelashes; (2) a muscle layer containing mainly the orbicularis oculi muscle, responsible for closing the lid; (3) a fibrous layer that gives the lid its mechanical stability, the main parts of which are the tarsal plates, which directly border the opening between the lids, called the palpebral opening; and (4) the inner layer of the lid, a portion of the conjunctiva. The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and lids, but allows a considerable amount of rotation of the eyeball in the orbit.

The conjunctiva lines the eyelids and then curves back over the surface of the eyeball, forming an outer covering to the front part of this and ending at the transparent area of ​​the eye, the cornea. The part which lines the lids is called the palpebral part of the conjunctiva; the part that covers the white of the eyeball is called the bulbar conjunctiva. Between the bulb beam and the palpebral conjunctiva there are two loose, redundant portions that form recesses that project back toward the equator of the globe. These recesses are called the upper and lower whores, or conjunctival sacs; it is the looseness of the conjunctiva at these points that allows movements of the eyelids and eyeball.

The fibrous layer, which gives the lid its mechanical stability, consists of the thick, and relatively rigid, tarsal plates, which directly border the palpebral opening, and the much thinner palpebral fascia, or sheet of connective tissue; the two together are called the septum orbitale. When the lids are closed, the entire opening of the track is covered by this septum. Two ligaments, the medial and lateral palpebral ligaments, attached to the orbit and to the septum orbitale, stabilize the position of the eyelids in relation to the globe. The medial ligament is by far the stronger.

Closure of the eyelids is achieved by contraction of the orbicularis muscle, a single oval skin muscle that extends from the forehead and face regions and surrounds the orbit in the eyelids. It is divided into orbital and palpebral portions, and it is essentially the palpebral portion, within the lid, that causes the closure of the lid. The palpebral portion crosses the eyelids from a ligament called the medial palpebral ligament and from the neighboring bone of the orbit in a series of half ellipses that meet outside the outer corner of the eye, the lateral canthus, to form a band to form fibers called the lateral palpebral raphe. Additional parts of the orbicularis have received separate names—namely, Horner’s muscle and the muscle of Riolan; they come into close contact with the lacrimal apparatus and help with the drainage of the tears. The muscle of Riolan, which lies close to the lid edges, contributes to keeping the lids in close apposition. The orbital portion of the orbicularis is not normally concerned with blinking, which can be performed entirely by the palpebral portion; however, it is about closing the eyes tightly. The skin of the forehead, temples and cheek is then drawn to the medial (nasal) side of the orbit, and the radiating furrows, which are formed by this action of the orbital portion, eventually lead to the so-called crow’s feet of elderly persons. It should be realized that the two sections can be activated independently; thus, the orbital portion may contract, causing a furrowing of the eyebrows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and allows the eyes to remain open.

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Opening of the eye is not only the result of passive relaxation of the orbicularis muscle, but is also the effect of the contraction of the levator palpebrae superioris muscle of the upper eyelid. This muscle originates with the extraocular muscles at the apex of the orbit as a narrow tendon and runs forward in the upper limb as a broad tendon, the levator aponeurosis, which attaches to the front surface of the tarsus and the skin covering the upper covered is attached. lid. Contraction of the muscle causes elevation of the upper eyelid. The nerve connections of this muscle are closely related to those of the extraocular muscle required to lift the eye, so that when the eye looks upward, the upper eyelid tends to move up in unison.

The orbicularis and levator are striated muscles under voluntary control. The eyelids also contain smooth (involuntary) muscle fibers that are activated by the sympathetic division of the autonomic system and tend to widen the palpebral fissure (the eye opening) by elevation of the upper, and depression of the lower lid.

In addition to the muscles already described, other facial muscles often cooperate in the act of closing or opening lids. Thus, the corrugator supercilii muscles draw the eyebrows toward the bridge of the nose, making a projecting “roof” over the medial corner of the eye and producing characteristic furrows in the forehead; the roof is mainly used to protect the eye from the glare of the sun. The pyramidalis, or procerus, muscles occupy the bridge of the nose; they arise from the lower part of the nasal bones and are attached to the skin of the lower part of the forehead on either side of the midline; they pull the skin in transverse furrows. In lid opening, the frontalis muscle, which originates high on the forehead, midway between the coronal suture, a suture across the top of the skull, and the orbital margin, is attached to the skin of the eyebrows. Contraction therefore causes the eyebrows to rise and against the action of the orbital portion of the orbicularis; the muscle is especially used when looking up. It is also brought into action when vision is made difficult, either by distance or the absence of sufficient light.

The outer layer of the lid is the skin, with features not very different from the skin on the rest of the body, with the possible exception of large pigment cells, which, although found elsewhere, are much more

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