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Apr 29, 2023 · Learn about the anatomy of the eye, from the outside to the inside, and how it works to give us clear vision. Explore the functions and structures of the eye's parts, such as the cornea, the iris, the pupil, the lens, the retina, and the optic nerve. See illustrations and diagrams of the eye's anatomy and how it relates to diseases and conditions that affect the eye.
- Overview
- The conjunctiva
- The fibrous layer
- The muscles of the lids
- The skin
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human eye, in humans, specialized sense organ capable of receiving visual images, which are then carried to the brain.
The conjunctiva lines the lids and then bends back over the surface of the eyeball, constituting an outer covering to the forward part of this and terminating at the transparent region of the eye, the cornea. The portion that lines the lids is called the palpebral portion of the conjunctiva; the portion covering the white of the eyeball is called t...
The fibrous layer, which gives the lid its mechanical stability, is made up of the thick, and relatively rigid, tarsal plates, bordering directly on the palpebral aperture, 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 whole opening of the orbit 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 lids in relation to the globe. The medial ligament is by far the stronger.
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Closure of the lids is achieved by contraction of the orbicularis muscle, a single oval sheet of muscle extending from the regions of the forehead and face and surrounding the orbit into the lids. It is divided into orbital and palpebral portions, and it is essentially the palpebral portion, within the lid, that causes lid closure. The palpebral portion passes across the lids from a ligament called the medial palpebral ligament and from the neighbouring 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 of fibres called the lateral palpebral raphe. Additional parts of the orbicularis have been given separate names—namely, Horner’s muscle and the muscle of Riolan; they come into close relation with the lacrimal apparatus and assist in drainage of the tears. The muscle of Riolan, lying close to the lid margins, contributes to keeping the lids in close apposition. The orbital portion of the orbicularis is not normally concerned with blinking, which may be carried out entirely by the palpebral portion; however, it is concerned with closing the eyes tightly. The skin of the forehead, temple, and cheek is then drawn toward the medial (nose) side of the orbit, and the radiating furrows, formed by this action of the orbital portion, eventually lead to the so-called crow’s feet of elderly persons. It must be appreciated that the two portions can be activated independently; thus, the orbital portion may contract, causing a furrowing of the brows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and allows the eyes to remain open.
Opening of the eye is not just the result of passive relaxation of the orbicularis muscle but also is the effect of the contraction of the levator palpebrae superioris muscle of the upper lid. This muscle takes origin with the extraocular muscles at the apex of the orbit as a narrow tendon and runs forward into the upper lid as a broad tendon, the levator aponeurosis, which is attached to the forward surface of the tarsus and the skin covering the upper lid. Contraction of the muscle causes elevation of the upper eyelid. The nervous 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 up in unison.
The orbicularis and levator are striated muscles under voluntary control. The lids also contain smooth (involuntary) muscle fibres 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 lid closure or opening. Thus, the corrugator supercilii muscles pull the eyebrows toward the bridge of the nose, making a projecting “roof” over the medial angle of the eye and producing characteristic furrows in the forehead; the roof is used primarily 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 portion 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 furrows. In lid opening, the frontalis muscle, arising high on the forehead, midway between the coronal suture, a seam 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 opposes the action of the orbital portion of the orbicularis; the muscle is especially used when one gazes upward. It is also brought into action when vision is rendered difficult either by distance or the absence of sufficient light.
The outermost layer of the lid is the skin, with features not greatly different from skin on the rest of the body, with the possible exception of large pigment cells, which, although found elsewhere, are much more numerous in the skin of the lids. The cells may wander, and it is these movements of the pigment cells that determine the changes in col...
Learn about the structure and function of the human eye, a specialized sense organ that receives visual images. Explore the orbit, eyelids, conjunctiva, cornea, iris, pupil, lens, and retina of the eye.
Feb 20, 2024 · Learn about the 16 parts of the eye and how they work together to enable vision. Find out how the cornea, iris, pupil, retina, and other structures affect your eyesight and what conditions can affect them.
Photoreceptors (Rods and Cones) - Eye Anatomy- Fovea of the Eye (Anatomy, Functions & Associated Conditions)
- Retina: Anatomy, Functions, and Conditions
- Iris of the Eye (Anatomy, Functions & Associated Conditions)
- Vitreous Humor - Anatomy & Function
- Optic Nerve - Function, Anatomy & Associated Conditions
- Optic Chiasm Function: Its Impact on Vision
- Anterior chamber. The front section of the eye's interior where aqueous humor flows in and out, providing nourishment to the eye.
- Aqueous humor. The clear watery fluid in the front of the eyeball.
- Blood vessels. Tubes (arteries and veins) that carry blood to and from the eye.
- Caruncle. A small, red portion of the corner of the eye that contains modified sebaceous and sweat glands.
Nov 15, 2023 · Learn how your eyes work, what parts they include and what can go wrong. Find out about refractive errors, corneal disorders, retinal disorders, optic nerve problems and more.
Apr 20, 2022 · Learn how the eyes work together to help you see, from light passing through the cornea and pupil to light hitting the retina and electrical signals traveling to the brain. Find out how tears, the iris, the lens, and the optic nerve also contribute to healthy vision.
May 10, 2022 · Learn about the anatomy, function, and diseases of the eye with a diagram and a list of eye conditions. Find out how to keep your eyes healthy and what to do if you have eye problems.
