Structures of the Eye
- A skeletal muscles allow it to open and close.
- Eyelashes help keep the dust out of the eyeball.
- There is a thin membrane lining called the conjunctiva, which folds over the white of the eye. Inflammation of the area is called conjunctivitis.
- Lacrimal glands, located at the upper outer corner of the eyeball, produce tears.
- Tears contain an enzyme called lysozyme that inhibits the growth of most bacteria.
- Two small openings into the superior and inferior lacrimal canals take tears to the lacrimal sac, and then on to the nasolacrimal duct, that empties tears into the nasal cavity.
- The orbit houses and protects the eyeball which is formed by the
- maxilla bone;
- zygomatic bone;
- frontal bone;
- sphenoid bone;
- ethmoid bone.
- Six extrinsic muscles attach the eye to the bony socket:
- inferior rectus;
- medial rectus;
- superior rectus;
- lateral rectus;
- inferior oblique;
- superior oblique.
The four rectus muscles move the eye up and down and from side to side. The two oblique muscles rotate the eye.
- Three cranial nerves are responsible for movement:
- oculomotor (3rd);
- trochlear (4th);
- abducens (6th).
- Three layers of the eyeball:
- outer sclera;
- middle choroid layer;
- inner retina.
The sclera is the thickest layer, made from fibrous connective tissue which is visible as the white of the eye. The choroid layer contains blood vessels and a dark blue pigment that absorbs light within the eyeball preventing glare. The retina lines the posterior two-thirds of the eyeball, and contains the visual receptors called rods and cones. Rods only detect the presence of light and are more numerous toward the edge (periphery) of the retina. Cones detect colors and are more numerous at the center of the retina.
- The lens is made up of a transparent, elastic protein that has no capillaries. Its shape is changed by the ciliary muscle enabling the eye to focus light from objects at varying distances.
- The fovea is a small depression directly behind the center of the lens. It contains only cones and is the area for the best color vision. Dim light or night vision is dependent on the rods at the sides of the visual fields.
- Ganglion neurons carry impulses generated by the rods and cones, and converge at the optic disc, which has no rods or cones. This is known as the blind spot. The neurons then pass through the wall of the eyeball to become known as the optic nerve.
- There are two cavities within the eye:
- posterior cavity
- anterior cavity
The posterior cavity is the larger of the two, found between the lens and the retina. It contains vitreous humor, a semi-solid substance that keeps the retina in place.
The anterior cavity is found between the front of the lens and the cornea and contains aqueous humor, the tissue fluid formed by capillaries in the ciliary body, flowing anteriorly through the pupil, and reabsorbed by the canal of Schlemm (small veins also known as the scleral venous sinus), found at the junction of the iris and the cornea. Aqueous humor has nourishing capabilities and an abnormal accumulation is called glaucoma.
- The anterior portion contains two parts:
- ciliary body;
- iris.
The iris is just in front of the lens, and is the colored portion of the eyeball. The central opening of the iris is called the pupil. Two sets of smooth muscle fibers (sphincter pupillae and dilator pupillae) change its diameter. A sympathetic response dilates the pupil through contractions of the radial fibers. A parasympathetic response constricts the pupil through contractions of circular fibers of the oculomotor nerve. Pupilary constriction is a reflex that protects the retina from intense light or permits more acurate vision
The ciliary body is a muscle that surrounds the edge of the lens connected to suspensory ligaments. The cornea is the most anterior portion, differing from the rest of the sclera in that it is transparent and has no capillaries. It is the first part of the eye to refract, or bend, light rays.
See a slideshow look inside the eye.
This page was updated in November 2005.
