Eye

An eye is a sensory organ that gathers optical information and converts it into neural signals. In the vertebrate eye, the cornea and crystalline lens focus light toward the retina. Image formation, sensory transduction and later visual interpretation are related but distinct stages.

The vitreous body is the transparent gel occupying most of the space behind the crystalline lens and in front of the retina. It contributes to the eye’s internal optical and structural environment. It is distinct from the more freely circulating aqueous fluid in the anterior eye.

The posterior chamber is the narrow aqueous-filled region behind the iris and in front of the crystalline lens and related structures. Aqueous fluid passes from it through the pupil. It is not the large vitreous-filled space behind the lens.

The choroid is the vascular and pigmented layer between the retina and sclera. It supports the outer retina and helps limit stray light. Its position distinguishes it from the retinal vessels supplying other parts of the sensory tissue.

The ciliary body is the anterior continuation of the vascular middle coat of the eye. It includes the ciliary muscle and processes involved in aqueous-fluid production. Connections to the lens through zonular fibers link its muscular action to accommodation.

The suspensory fibre apparatus transmitting forces between the ciliary body and the capsule of the crystalline lens. Individual zonular fibres are its components.

A nictitating membrane is a movable third eyelid present in various vertebrates. It can protect or moisten the exposed eye while allowing some visual access. Its presence and mobility vary among lineages, so it is not a universal feature of every eye.

Miosis is constriction of the pupil. In the human eye, contraction of the sphincter pupillae muscle reduces the opening in the iris. Pupil size is distinct from lens focus, even though pupil constriction and accommodation can occur together.

A contact lens is an optical device placed on the front surface of the eye. Its shape and refractive properties alter image formation, for example to correct refractive error. Unlike an implanted lens, it remains external to the internal ocular chambers.

A toric contact lens has different optical powers along different principal meridians, allowing correction of astigmatism. Its orientation matters because rotating those meridians changes the direction of correction. Optical power and rotational stability are therefore separate design requirements.

An optical section through the visual apparatus in a specified direction. The two principal meridians of regular astigmatism have different powers and are perpendicular.

Extorsion of the eye rotates the superior pole of the eyeball away from the nose. It is rotation around an approximately front-to-back axis, distinct from horizontal abduction of the eye. Several extraocular muscles can contribute depending on the gaze position.

Intorsion of the eye rotates the superior pole of the eyeball toward the nose. It is the torsional counterpart of extorsion of the eye. The motion should be distinguished from simply directing the visual axis medially.

Abduction of the eye moves the visual axis away from the nose. The lateral rectus muscle is the main muscle for this horizontal action. It differs from torsional rotation of the eyeball around the visual axis.

Adduction of the eye moves the visual axis toward the nose. The medial rectus muscle is the main muscle for this horizontal action. Position changes the pulling geometry of other extraocular muscles, making muscle actions depend partly on initial gaze direction.

The lacrimal sac receives tears draining from the eyelid canaliculi and continues into the nasolacrimal duct. It belongs to the drainage pathway. The lacrimal gland, by contrast, contributes secreted fluid to the exposed ocular surface.

The lacrimal gland is an orbital gland contributing fluid to the tear film. Its ducts deliver secretion toward the eye surface. Production, distribution by eyelid movement and drainage through the lacrimal sac are separate parts of tear handling.

An eyelid is a movable protective fold covering the exposed eye. Blinking distributes the tear film and helps clear the surface. The tarsal plate provides support, while associated muscles control opening and closure.

A tarsal plate is a dense connective-tissue support within an eyelid. It contains specialized glands and helps maintain lid shape. It is not a plate of bone, despite the stiffness implied by its name.

The iris is the adjustable diaphragm around the pupil. Its muscles alter the opening through which light enters the eye, while its pigment contributes to visible eye color. Changing pupil aperture and changing lens curvature are distinct optical controls.

The iris dilator muscle has a roughly radial arrangement that enlarges the pupil when activated. Its action opposes that of the sphincter pupillae muscle. Autonomic control links the size of the optical opening to physiological state.

The opening in the iris through which light enters the eye. Its aperture affects admitted pencils of rays; the cornea and crystalline lens provide focusing power.

Dilatation of the pupil; it is a sign requiring interpretation in context, rather than an identification of a unique poison.

The sphincter pupillae muscle is the circular smooth muscle of the iris that constricts the pupil. Its activation produces miosis. It differs from the radial dilator both in fiber arrangement and in its autonomic control pathway.

The cornea is the transparent anterior surface of the eye and an important refracting interface. Its curvature strongly affects focus. Transparency, surface smoothness and mechanical protection must coexist in the same tissue, while the crystalline lens supplies a separately adjustable optical component.

The crystalline lens is the transparent internal optical structure behind the iris. Changing its curvature during accommodation changes focusing power. Its refractive role is distinct from controlling the light aperture, which is performed by the pupil.

A ring of smooth muscle in the ciliary body. Its contraction alters the geometry of the ciliary zonule and permits the crystalline lens to become more curved for near ocular accommodation.

A suspensory fibre transmitting tension to the crystalline lens. Reduced zonular tension during near ocular accommodation permits the crystalline lens to become more rounded.

Adjustment of the eye's optical power to maintain a focused image on the retina as object distance changes. For near vision the ciliary muscle contracts, tension in the zonular fibres decreases and the elastic crystalline lens becomes more rounded.

The tough outer coat of the eye, providing attachments for extraocular muscles.

The scleral spur is an inward projection of scleral tissue near the anterior chamber angle. It provides an attachment and landmark for nearby ciliary and drainage structures. Its importance follows from those spatial relationships rather than from forming a separate optical surface.

An extraocular muscle rotates the eye through its attachment to the sclera. The six globe-moving muscles have position-dependent horizontal, vertical and torsional actions.

The levator palpebrae superioris elevates the upper eyelid. It is controlled chiefly through the oculomotor nerve, with additional smooth-muscle contributions to lid position. Raising the lid and rotating the eyeball are different movements even though both affect visible gaze.

The inferior oblique muscle is an extraocular muscle contributing to extorsion, elevation and abduction of the eye. Its effect depends on gaze position; elevation is especially informative when the eye is adducted. Its course differs from the rectus muscles arising posteriorly in the orbit.

The superior oblique muscle passes through the trochlea of superior oblique before attaching to the eyeball. It contributes to intorsion, depression and abduction, depending on gaze position. It is supplied by the trochlear nerve.

The trochlea of superior oblique is a pulley-like structure in the anterosuperomedial orbit. It redirects the tendon of the superior oblique muscle. The redirection explains why the muscle’s effective pull differs from a straight line between its belly and the eyeball.

The inferior rectus muscle primarily depresses the eye and also contributes to extorsion and adduction. It is supplied by the oculomotor nerve. Its vertical action is tested differently from depression supplied by the superior oblique muscle because the pulling geometry differs.

The superior rectus muscle primarily elevates the eye and also contributes to intorsion and adduction. It is supplied by the oculomotor nerve. Its line of pull explains why an extraocular muscle need not produce only the direction suggested by its name.

The lateral rectus muscle abducts the eye. It is supplied by the abducens nerve, unlike most other muscles rotating the eyeball. Its relatively direct horizontal action provides a useful anatomical example connecting one muscle, one nerve and one principal movement.

The medial rectus muscle adducts the eye and is supplied by the oculomotor nerve. Coordinated activity with the opposite eye supports convergence. A local muscle action and the binocular control problem are therefore different explanatory levels.