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Parts Of The Eye And Their Function

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

Eyes: How They Work, Anatomy & Common Conditions

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

The Editors of Encyclopedia The editors of Encyclopedia oversee subject areas in which they have extensive knowledge, either through years of experience gained working on that content or through higher education. They write new content and verify and edit the content they receive from contributors.

Human eye, in humans a specialized sense organ that can receive visual images, which are then transported to the brain.

The eye is protected from mechanical injury by being enclosed in an eye socket, which consists of portions of several skull bones, forming a four-sided pyramid, the apex of which points toward the back of the head. The floor of the orbit is thus composed of parts of the maxilla, zygomatic, and palatine bones, while the roof consists of the orbital plate of the frontal bone and, behind it, by the lesser wing of the sphenoid bone. The optic foramen, the opening through which the optic nerve passes back to the brain and the great ophthalmic artery enters the orbit, is located on the nasal side of the apex; the superior orbital fissure is a larger hole through which large veins and nerves pass. These nerves can carry non-visual sensory messages, for example pain, or they can be motor nerves that control the eye muscles. There are other fissures and channels that transmit nerves and blood vessels. The eyeball and its functional muscles are surrounded by a layer of orbital fat that acts somewhat like a cushion and allows smooth rotation of the eyeball around a nearly fixed point, the center of rotation. The bulging of the eyeballs (proptosis) in exophthalmic goiter is caused by the accumulation of fluid in the orbital fatty tissue.

Parts Of The Eye And Their Functions

It is crucial that the front surface of the eyeball, the cornea, remains moist. This is accomplished by the eyelids, which during waking hours sweep the secretions of the lacrimal apparatus and other glands over the surface at regular intervals, and during sleep they cover the eyes and prevent evaporation. The lids have the additional function of preventing injuries from foreign objects through the action of the blink reflex. The eyelids are essentially folds of tissue that cover the front of the eye socket and, when the eye is open, leave an almond-shaped opening. The points of the almond are called canthi; that closest to the nose is the inner corner of the eye, and the other is the outer corner of the eye. The lid can be divided into four layers: (1) the skin, with glands opening onto the surface of the edge of the lid, and the eyelashes; (2) a muscle layer mainly containing the orbicularis oculi muscle, responsible for closing the lid; (3) a fibrous layer that gives the lid its mechanical stability, the most important parts being the tarsal plates, which are immediately adjacent to the opening between the lids, called the palpebral opening; and (4) the inner layer of the eyelid, part of the conjunctiva. The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and eyelids, but allows a significant degree of rotation of the eyeball within the orbit.

The conjunctiva lines the eyelids and then curves back over the surface of the eyeball, forming an outer covering for the front part of it and ending at the transparent area of ​​the eye, the cornea. The portion that lines the eyelids is called the palpebral portion of the conjunctiva; the part that covers the white of the eyeball is called the bulbar conjunctiva. Between the bulbar and the palpebral conjunctiva are two loose, redundant portions that form recesses that project posteriorly toward the equator of the globe. These recesses are called the superior and inferior fornices or conjunctival sacs; it is the looseness of the conjunctiva at these points that allows movements of the eyelids and globe.

The fibrous layer, which gives the lid its mechanical stability, consists of the thick and relatively stiff tarsal plates, which are immediately adjacent to 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 orbit is covered by this septum. Two ligaments, the medial and lateral palpebral ligaments, attached to the orbit and to the orbital septum, stabilize the position of the eyelids relative to the globe. The medial ligament is by far the stronger.

Closing of the eyelids is achieved by contraction of the orbicularis muscle, a single oval muscle tissue that extends from the areas of the forehead and face and surrounds the eye socket into the eyelids. It is divided into orbital and palpebral parts, and it is essentially the palpebral part, within the lid, that causes lid closure. The palpebral portion runs across the eyelids from a ligament called the medial palpebral ligament and from the adjacent bone of the orbit in a series of semi-ellipses that meet outside the outer corner of the eye, the lateral canthus, to form a band of fibers to form the lateral palpebral raphe. Other parts of the orbicularis have been given separate names, namely Horner’s muscle and Riolan’s muscle; they are closely related to the lacrimal apparatus and aid in the drainage of tears. Riolan’s muscle, which is close to the edges of the eyelids, helps keep the eyelids close together. The orbital portion of the orbicularis is not normally concerned with blinking, which may be performed entirely by the palpebral portion; however, it involves closing the eyes tightly. The skin of the forehead, temple and cheek is then drawn towards the medial (nasal) side of the orbit, and the radiating grooves formed by this action of the orbit portion eventually lead to the so-called crow’s feet of the elderly. . It should be understood that the two parts can be activated independently; the orbital portion can thus contract, creating a frown in the eyebrows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and the eyes can remain open.

Parts Of The Eye And Function

Opening of the eye is not only the result of passive relaxation of the orbicularis muscle, but also of the contraction of the levator palpebrae superioris muscle of the upper eyelid. This muscle originates in the extraocular muscles at the apex of the eye socket as a narrow tendon and runs anteriorly into the upper eyelid as a broad tendon, the levator aponeurosis, which attaches to the anterior surface of the tarsus and the skin covering it. top part covered. 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 elevate the eye, so that when the eye looks upward, the upper eyelid tends to move upward at the same time.

The orbicularis and levator are striated muscles that are 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 raising the upper and depressing the lower eyelid.

In addition to the muscles already described, other facial muscles often participate in closing or opening the lid. For example, the corrugator supercilii muscles draw the eyebrows towards the bridge of the nose, creating a protruding ‘roof’ over the medial corner of the eye and creating characteristic furrows in the forehead; the roof is mainly used to protect the eye from the sun’s glare. The pyramidal 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 into transverse grooves. When opening the lid, the frontalis muscle, which originates high on the forehead, midway between the coronal suture, a seam across the top of the skull and the orbital rim, is attached to the skin of the eyebrows. Contraction therefore causes the eyebrows to rise and counteracts the action of the orbital part of the orbicularis; the muscle is mainly used when looking upwards. It is also used when vision is impaired by distance or the absence of sufficient light.

The outer layer of the eyelid is the skin, with features not much 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 fragile.

The Eye (human Body): Kathleen Elgin: 9780851660929: Amazon.com: Books

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