Eye

The eyes are considered the most important sensory organ because 90% of the information about the environment reaches the brain from the eyes. The external structures of the eye are shown in Fig. 20-1. The orbital cavity is composed of bones that protect and adipose tissue that cushions the eye. When the eyelids and eyelashes close or “blink,” they protect the eye from injury. The conjunctiva is a mucous membrane that protects and lubricates the eyelids and part of the eye. The lacrimal apparatus forms tears that keep the eye moist and lubricated. Movement of the eye is controlled by the extrinsic muscles. Only one fifth of the eye is actually exposed to the environment.

The internal structures of the eye are also shown in Fig. 20-1. The sclera is a tough, white tissue that supports and gives structure to the eye. The sclera is continuous with the transparent cornea that covers the iris and pupil. The cornea focuses light rays on the retina at the back of the eye. The blood supply for the eye originates in the choroid, iris, and ciliary muscles. The iris and ciliary muscles are the intrinsic muscles of the eye.

The eyeball is not solid but is divided into sections, the anterior and posterior cavities. The anterior cavity is filled with a clear watery fluid called aqueous humor. The posterior cavity is filled with a semisoft gelatin-like substance called vitreous humor. Both the aqueous and vitreous humor help maintain the shape of the eye.

The opening called the pupil is in the choroid layer. The iris is a round, colored muscle that surrounds the pupil. The iris contracts and relaxes to adjust the amount of light entering the eye through the pupil. The lens is a convex, transparent tissue located directly behind the pupil that focuses and directs incoming light on the retina of the eye. The amount of light admitted into the eye is regulated by the movement of the iris.

Vision is similar to the action of a camera, beginning with refraction, the process of the lens bending light rays as they enter the eye to focus on the retina (Fig. 20-2). The eye changes the shape of the lens to focus near and far through the process of accommodation. The pupil constricts to focus the object on the retina and protect it from receiving too much light. The eyes converge on the object so that single binocular vision occurs and only one object is seen.

Specialized cells called rods and cones in the retina absorb the light. The retina of each eye contains 100 million rods and 7 million cones. Rods are sensitive to dim light. Cones react to bright light and allow color distinction through three types of pigments that are sensitive to different wavelengths of light. The three photopigments recognize the colors: green, red, and blue. The impulses released by the pigments in the rods and cones are transmitted to the brain by the optic nerve.

Ear

The auditory or acoustic sense (hearing) is the primary function of the ear. A second function of the ear is to help maintain equilibrium. The ear has three parts, called the external, middle, and inner ear (Fig. 20-3).

The external ear channels the incoming sound waves or vibrations. The auricle or pinna is the flap of tissue on the side of the head that collects and transmits sound waves through the ear or auditory canal to the eardrum (tympanic membrane). Specialized glands in the ear canal produce earwax (cerumen) that protects the middle ear from entry of foreign particles.

The middle ear is an air-filled chamber that begins with the tympanic membrane, which changes sound waves into mechanical movements. Auditory bones (ossicles) transmit the sound vibrations. These three bones are called the hammer (malleus), anvil (incus), and stirrup (stapes). The ossicles amplify and transmit the sound to the inner ear. Two openings into the inner ear are the membrane-covered round window and the oval window, which touches the stapes. Another opening between the middle ear and pharynx is called the eustachian tube. The eustachian tube has two main functions. It allows the pressure of air in the middle ear to be equalized with the air pressure of the environment. Additionally, fluids and mucus from the middle ear are drained to the nasopharynx. Swallowing and yawning open the eustachian tube for these purposes.

The inner ear contains a series of canals called bony labyrinth, which includes the cochlea, semicircular canals, and vestibule with the membranous labyrinth inside it. The movement of fluid and hair cells lining the cochlea converts the mechanical vibrations from the ossicles to neural impulses. Each ear contains 16,000 hair cells. Each hair cell has 100 stereocilia or bristles that transmit impulses to the auditory cranial nerves. The semicircular canals contain a clear fluid called endolymph that gives a sense of balance when the body is in motion. Two chambers called the saccule and utricle of the vestibule maintain static or resting equilibrium.

Hearing is a result of interpretation of sound waves. Sound waves are described by their amplitude (volume) and pitch (frequency). A sound is perceived when the tympanic membrane is vibrated or the hairs in the cochlea are stimulated. Sound waves may be transmitted through the air, bone, or fluid.

Tongue

Taste, or the gustatory sense, is perceived by specialized cells located in projections (papillae) on the tongue called taste buds. Taste buds are chemoreceptors. The five tastes perceived by the tongue are sweet, sour, bitter, salty, and umami (Fig. 20-4). Umami, described as “meatiness,” is a response to glutamic acid found in processed meats, cheese, and many Asian foods. Each taste bud has 50 to 100 taste cells. These cells respond to all five types of taste. All areas of the tongue sense all five of the tastes. As more research is completed, new “tastes” may be identified. It is possible that “metallic” is also a primary taste. Before a taste can be sensed, the substance must be dissolved in fluid or saliva. The particular flavor of an item is identified by smell as well as taste.

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