From Canvas (Eyes Anatomy Vocabulary) - Unit 13: Eyes and Ears

Ophthalmology

The medical specialty that studies the anatomy and physiology of the eye and uses diagnostic tests, medical and surgical procedures, and drugs to treat eye diseases.

Ophthalmologist

A medical specialist who focuses on the anatomy and physiology of the eye and uses diagnostic tests, medical and surgical procedures, and drugs to treat eye diseases.

Ophthalmoscope

An instrument used to examine the interior structures of the eye.

Optometrist

A healthcare specialist who focuses on measuring the accuracy of vision to determine whether corrective lenses are needed.

Vision Mechanics

Vision is made possible through the coordinated actions of nerves that control the movement of the eyeball, the amount of light admitted by the pupil, the focusing of that light on the retina by the lens, and the transmission of the resulting sensory impulses to the brain by the optic nerve.

Visual Pathway Step 1: Entry & Primary Refraction

Light rays bounce off an object (e.g., a tree) and enter the eye through the cornea (the clear, transparent portion of the outer eye coating), which performs most of the initial bending (refraction) of light rays to begin focusing them.

Visual Pathway Step 2: Pupil & Iris Regulation

Light passes through the opening of the iris called the pupil; the iris muscles dilate or constrict the pupil to control the amount of light entering (shrinking to pinhead size in bright light to prevent overload, and enlarging in dim light).

Visual Pathway Step 3: Lens & Accommodation

Light reaches the crystalline lens, where ciliary body muscles contract or relax to change the shape of the lens via suspensory ligaments (increasing curvature/rounding for near objects, flattening for distant objects) to fine-tune the focus.

Visual Pathway Step 4: Vitreous Humor & Retina Projection

Light rays pass through the clear, jellylike vitreous humor in the posterior cavity and land on the retina. Light from objects directly in the line of vision lands on the macula lutea (fovea centralis) for the sharpest image, while peripheral light lands on the outer retina.

Visual Pathway Step 5: Retinal Image & Phototransduction

The image projected onto the retina by the bent light rays is upside down and backwards. Photoreceptors (rods for dim light/peripheral vision and cones for bright light/color) convert this light energy into electrical nerve impulses.

Visual Pathway Step 6: Transmission to the Brain

Nerve impulses travel from the retina, pass through the optic disc (the blind spot which lacks rods and cones and serves as the entry point for the retinal artery), and travel via the optic nerve to the brain, where the upside-down image is interpreted as upright vision.

Refraction

The process of bending light rays as they pass through the various transparent structures of the eye to produce a clear image on the retina.

Errors of Refraction

Visual impairments that occur when the eyeball is abnormally shaped or when the lens loses its ability to accommodate to near vision, resulting in blurred vision that can typically be corrected with lenses.

Path of Light and Detailed Visual Pathway

Light rays bounce off objects and enter the eye through the transparent cornea, which does most of the light refraction. Light then passes through the pupil in the iris, where muscles contract or dilate to regulate light entry (shrinking in bright light and enlarging in dim light). Next, light reaches the biconvex crystalline lens, where ciliary muscles contract or relax to change the shape of the lens via suspensory ligaments to fine-tune the focus. The light rays pass through the clear, jellylike vitreous humor in the posterior cavity and land on the retina, where central objects direct light to the macula and peripheral objects fall on the outer retina. The light creates an image on the retina that is upside down and backwards. Photoreceptive nerve cells (rods and cones) convert this light energy into electrical nerve impulses. These impulses travel through the optic disc into the optic nerve, which transmits them to the brain where the image is interpreted as sight.

Orbit

An external cavity of the front skull that houses and protects the eyeball. It is a cone-shaped cavity formed by the combination of several bones, lined with a mass of orbital fat for cushion and insulation, and containing several foramina (openings) for blood vessels and nerves, the largest being the optic foramen for the optic nerve and ophthalmic artery.

Eye Muscles

External structural muscles attached to the eye that help maintain the overall shape of the eyeball and direct its movement.

Eyelids

External structures continuous with the skin that cover the eyeball, keeping the surface continuously lubricated and protected from dust, debris, intense light, and physical impact through the blinking motion.

Eyelashes

External hair structures located along the edges of the eyelids that are highly sensitive, providing a warning response to trigger blinking and preventing foreign materials or insects from contacting the eyeball surface.

Conjunctiva

A thin, colorless, protective mucous membrane layer that lines the anterior part of the eye and the inner surface of the eyelids; it appears white because it covers the underlying sclera.

Lacrimal Gland

Almond-shaped tear glands located at the upper outer edge of each eye that continuously produce an aqueous fluid (tears) containing the antibacterial enzyme lysozyme to clean, protect, and lubricate the conjunctival surface.

Lacrimal Duct

A short tube located at the inner corner of the eyelid through which tears drain out of the eye and into the nose or nasolacrimal duct.

Nasolacrimal Duct

The anatomical passageway that drains lacrimal fluid from the eye into the nasal cavity, with its enlarged upper portion known as the lacrimal sac.

Sclera

The outermost, tough, fibrous layer that forms the white portion of the eye, giving the eyeball its structural shape and serving as a protective covering. It is thinnest over the anterior surface and thickest at the back, where its posterior surface connects to the outer covering of the optic nerve.

Cornea

The transparent, nonvascular anterior portion of the fibrous outer layer that covers the colored part of the eye; its curved, non-vascular surface performs the majority of light refraction to focus light onto the retina.

Iris

The colored middle layer of the eye visible beneath the cornea, composed of connective tissue and muscle tissue. Its color is determined by melanin density (brown eyes have high pigment, blue eyes have low pigment), and its internal muscles regulate the diameter of the central opening (the pupil).

Pupil

The adjustable central opening in the iris that allows light to reach the retina, contracting in bright light to restrict light entry and dilating in dark settings to allow more light to enter.

Choroid

The highly vascular middle membrane located directly beneath the sclera and between the retina and sclera. It is pigmented to prevent internal reflection of light, contains extensive capillaries to provide blood and nutrients to the back of the eye, attaches to the optic nerve edges, and encompasses the iris, ciliary body, and suspensory ligaments.

Ciliary Body

A thickened portion of the vascular middle layer located on each side of the lens to which the iris attaches; it contains smooth muscles that alter lens shape for near vision focus and secretes aqueous humor.

Suspensory Ligaments

Fibril strands that radiate from the ciliary body and attach to the lens, holding the lens securely in place and assisting in changing its curvature during focusing.

Retina

The innermost, light-sensitive nerve cell layer of the eye that contains photoreceptors (rods and cones) to convert light rays into electrical nerve impulses and transmit them to the brain via the optic nerve.

Rods

Photoreceptive nerve cells in the retina (roughly 120 million per eye) that are highly sensitive to low light and responsible for peripheral and dim-light vision. They contain rhodopsin for night vision, react to a single photon to produce black-and-white images, and function in both day and night settings without perceiving color.

Cones

Photoreceptive nerve cells in the retina (roughly 6 million per eye concentrated in the macula) that require multiple photons to activate, enabling sharp central vision, color perception (red, green, and blue sensitive cones), and detail processing in bright light settings.

Macula Lutea

An oval, yellowish spot located near the center of the retina that contains the vast majority of the eye's 6 million color-sensing cone cells.

Fovea Centralis

A tiny depression located within the macula lutea that contains exclusively cone cells, serving as the central focusing point of the eye to produce the sharpest visual image (central vision).

Optic Disc

The point of entry for the retinal artery and the point of exit for the optic nerve on the inner retina; because it lacks rods and cones entirely, it is insensitive to light and is known as the "blind spot."

Optic Nerve

The cranial nerve cable attached to the back of the eye that receives electrical sensory impulses from the retina and transmits them to the brain for visual interpretation.

Lens

A transparent, biconvex, colorless structure suspended by ligaments directly behind the iris that fine-tunes the refraction of light rays onto the retina.

Accommodation

The ability of the lens to change its shape for clear vision at varying distances; ciliary muscles contract to make the lens rounder/thicker for near objects and relax to make the lens flatter/thinner for distant objects.

Anterior Cavity

The front section of the eye (divided by the coronal plane relative to the lens) that contains two chambers filled with flowing aqueous humor, which maintains intraocular pressure and aids in light refraction.

Anterior Chamber

The space within the anterior cavity located directly in front of the lens and iris, filled with circulating aqueous humor.

Posterior Chamber

The space within the anterior cavity located directly behind the iris and in front of the lens, filled with circulating aqueous humor that flows between chambers.

Posterior Cavity

The large space located posterior to the lens, filled with a clear, non-reproducing, jellylike substance called vitreous humor that maintains eyeball shape and helps refract light (loss of this fluid can lead to blindness).

Life Span Considerations: Baby Development

Eye development begins as an outgrowth of the embryonic forebrain at 4 weeks and is structurally complete by 24 weeks. Eyebrows, eyelashes, and opening eyelids appear at 28 weeks. Newborn visual acuity is around 20/400, eyes often appear crossed due to underdeveloped eye muscles, and iris color appears blue/gray until permanent pigment fixes between 6 and 12 months. Tears do not form until 1 to 3 months due to immature lacrimal ducts. Depth perception develops around 9 months, visual acuity improves to 20/30 or 20/20 by age 2 or 3, and children typically remain farsighted until about 5 years of age.