Accessory structures of the eye

The accessory structures of the eye—eyebrows, eyelashes, eyelids, and lacrimal apparatus—function mainly as protective devices. In addition, six extrinsic eye muscles control gross eye movement and enable the eye to focus on any object in the visual field. The eye muscles are attached to the sclera (or white part of the eye) and move the eye laterally, medially, superiorly, and inferiorly.

The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids and the anterior surface of the eyeball to the edge of the cornea. Sometimes the blood vessels of the conjunctiva become dilated because of irritation or congestion, and the individual is said to have “bloodshot” eyes. The lower conjunctival sac is where eyedrops and eye ointment medication are usually administered.

Structure of the eyeball

The cornea is the central anterior portion of the sclera. It is transparent and covers the iris, which is the colored portion of the eye. The cornea allows light rays to enter the inner portion of the eye. The cornea is the first part of the eye that refracts (bends) light rays. It is dense, uniform in thickness, and nonvascular, and it projects like a dome beyond the sclera. The cornea is one of the most highly developed, sensitive tissues in the body and is innervated by the trigeminal nerve (cranial nerve V). The avascular cornea obtains oxygen primarily through absorption from the tear film layer that bathes the epithelium. A small amount of oxygen is obtained from the aqueous humor (watery fluid in front of the lens in the anterior chamber of the eye) through the endothelial layers. The degree of corneal curvature varies in different individuals and in the same person at different ages. The curvature is more pronounced in youth than in advanced age.

At the junction of the sclera and cornea is a special structure called the canal of Schlemm. This tiny venous sinus at the angle of the anterior chamber of the eye drains the aqueous humor and funnels it into the bloodstream. This aids in controlling intraocular pressure (IOP; the pressure within the eyeball).

The middle layer of the eyeball is the vascular tunic. It contains the choroid, the ciliary body, and the iris. The posterior portion of the vascular tunic is the choroid, which is a thin, dark brown membrane that lines most of the internal area of the sclera. It is highly vascular and supplies nutrients to the retina. The anterior portion of the vascular tunic forms the ciliary body, which is an intrinsic muscular ring that holds the lens in place and changes its shape for near or distant vision. The ciliary body also attaches to the iris, a pigmented intrinsic muscular ring that resembles a doughnut. Located slightly nasal to the center of the iris is a circular opening called the pupil. The iris lies between the cornea and the lens and regulates the amount of light entering the eye through the pupil, much like a camera shutter. Two sets of smooth muscle control the iris, which in turn controls the pupil. In bright light the circular muscle fibers of the iris contract and the pupil contracts; in dim light the radial muscles contract and the pupil dilates. Papillary constriction is a reflex that protects the retina from intense light or that permits more acute near vision.

The innermost tunic of the eye is the retina, a 10-layer, delicate, nervous-tissue membrane that receives images of external objects and transmits impulses through the optic nerve to the brain. It lies on the posterior portion of the eyeball. The retina contains specialized sensory cells called rods and cones (photoreceptors). The rods and cones are scattered throughout the retina except where the optic nerve exits the eye; this area is called the optic disk or blind spot. Rods are receptors for night vision and are also responsible for peripheral vision. Cones are responsible for day vision. The three kinds of cones are each sensitive to a different color: red, green, or blue. Color pigments that are sensitive to light enable the rods and cones to function. Rods detect only the presence of light, whereas cones detect different wave lengths of color.

The center of the retina is the fovea centralis, a pinpoint depression composed only of densely packed cones. The fovea centralis contains the greatest concentration of cones of any area in the retina. This area of the retina provides the sharpest visual acuity and most acute color vision. Surrounding the fovea is the macula, an area of less than 1 mm² that has a high concentration of cones and is relatively free of blood vessels. Vitamin A is responsible for the production of these color pigments. The absence of these three types of cones causes color blindness, which is an inherited condition found primarily in males.

Chambers of the eye

The eye is divided into the anterior and posterior chambers by the crystalline lens, a transparent, colorless structure that is biconvex, enclosed in a capsule, and held in place just behind the pupil by the suspensory ligament. The crystalline lens focuses light rays so that they form a perfect image on the retina. Anterior to the crystalline lens is the anterior chamber, which is filled with aqueous humor, a clear, watery fluid similar to blood plasma. The ciliary bodies of the choroid constantly secrete, drain, and replace aqueous humor to maintain normal IOP. Aqueous humor also helps maintain the eyeball’s shape, keeps the retina attached to the choroid, and refracts light.

The posterior chamber is filled with vitreous humor, a transparent, jellylike substance that gives shape to the eyeball, keeps the retina attached to the choroid, and refracts light. It differs from the aqueous humor in that it is not continuously replaced.

External ear

The external ear is composed of the auricle (pinna) and the external auditory canal. The canal is shaped like a small, curved tube (about 1 inch [2.5 cm] in length). It extends into the temporal bone, ending at the tympanic membrane—a thin, semitransparent membrane. The tympanic membrane separates the external ear from the middle ear and transmits sound vibrations to the internal ear by means of the auditory ossicles. The external ear is designed to collect sound waves and channel them to the middle ear. The upper part of the pinna is composed of elastic cartilage, whereas the lower part, the lobe, is mainly fleshy tissue. The whole structure is attached to the head by ligaments and muscles.

The walls of the external auditory canal are composed of cartilage-lined bone. The external auditory canal contains cilia (tiny hairs) and specialized sebaceous (oil) glands called ceruminous glands. They secrete cerumen (earwax), which protects the lining from infection. The cilia, in combination with the cerumen, also prevent foreign objects from entering the ear.

Middle ear

The middle ear, or tympanic cavity, is a small, air-filled chamber located within the temporal bone. The eustachian tube, or auditory canal, is lined with a mucous membrane that joins the nasopharynx and the middle-ear cavity. During swallowing or yawning, the tube allows air to enter the middle ear, which equalizes the air pressure on either side of the tympanic membrane. Because the pharynx, the eustachian tube, and the middle ear are all covered with a continuous mucous membrane, infection can travel easily from the throat to the middle ear. This is often seen in young children. The posterior wall of the middle ear opens into the mastoid process, an area filled with air spaces, which also aids in equalizing air pressure. Infection of the middle ear, if untreated, can spread to the mastoid process.

Extending along the middle-ear chamber are three small bones (ossicles) that carry sound waves from the external ear to the inner ear. These ossicles are named according to their shape: the malleus (hammer), the incus (anvil), and the stapes (stirrup). The internal surface of the tympanic membrane is connected to the first of these three bones, the malleus. The malleus transfers sound waves to the incus, which in turn transfers them to the stapes. The stapes pushes against the oval window, a small membrane that marks the beginning of the inner ear. When sound waves cause the tympanic membrane to vibrate, that vibration is transmitted and amplified by the ear ossicles as it passes through the middle ear. Movement of the stapes against the oval window causes movement of fluid in the inner ear.

Inner ear

Taste and smell

The receptors for the sense of smell (olfactory receptors) are located in the roof, or the upper part of the nasal cavity. On inhalation, an odor comes in contact with the olfactory receptors and the message is sent to the brain. Certain odors are remembered for a long time and stimulate certain memories (e.g., pine scent reminds people of Christmas; talcum powder reminds people of infants). The body is not able to regenerate olfactory cells; once they are damaged, the sense of smell is impaired.

Touch

The receptors for touch (tactile receptors) are located throughout the integumentary system. They respond to touch, pressure, and vibration.

Position and movement

Proprioception (sense of position) maintains the proper position of the body. Proprioceptors include any sensory nerve ending—such as those located in muscles, tendons, and joints—that responds to stimuli originating from within the body regarding movement and spatial position. They work in conjunction with the semicircular canals and the vestibule of the inner ear to maintain proper coordination. They orchestrate the body’s movements in running, walking, dancing, and many other activities. Once they receive information from the environment, they send it to the cerebellum for interpretation. Proprioceptors enable one to sense the position of the different parts of the body and be aware of the movement of each.