The middle ear is a cavity in the temporal bone, lined with epitheliums. It contains three auditory ossicles the malleus (hammer), the incus (anvil), and the stapes (stirrup)which transmit vibrations from the tympanic membrane to the fluid in the inner ear. The malleus is attached to the inner surface of the eardrum and connects with the incus, which in turn connects with the stapes. The base of the stapes is attached to the oval window (fenestra ovalis), the membrane-covered opening of the inner ear. These tiny bones link together to span the middle ear. They are suspended from its bony wall by ligaments and provide the mechanical means for transmission of sound vibrations to the inner ear.
The eustachian tube connects the middle ear with the pharynx. It is lined with a mucous membrane and is about 36 mm long. Its function is to equalize internal and external air pressure. For example, while riding an elevator in a tall building, you may experience a feeling of pressure in the ear. This is usually relieved by swallowing, which opens the eustachian tube and allows the pressurized air to escape and equalize with the area of lower pressure. Divers who ascend too fast to allow pressure to adjust may experience rupture of their eardrums. The eustachian tube can also be a pathway for infection of the middle ear.
The inner ear is filled with a fluid called endolymph. Sound vibrations that cause the stapes to move against the oval window create internal ripples that run through the endolymph. These pressurized ripples move to the cochlea, a small snail-shaped structure housing the organ of Corti, the hearing organ. The cells protruding from the organ of Corti are stimulated by the ripples to convert these mechanical vibrations into nerve impulses, which are relayed through the cochlear (8th cranial) nerve to the auditory area of the cortex in the temporal lobe of the brain. Here they are interpreted as the sounds we hear.
Other structures of the inner ear are the three semicircular canals, situated perpendicular to each other. Movement of the endolymph within the canals, caused by general body movements, stimulates nerve endings, which report these changes in body position to the brain, which in turn uses the information to maintain equilibrium.
The round window (fenestra rotunda) is another membranecovered opening of the inner ear. It contracts the middle ear and flexes to accommodate the inner ear ripples caused by the stapes.
Until the beginning of the last century, touch (feeling) was treated as a single sense. Thus warmth or coldness, pressure, and pain, were thought to be part of a single sense of touch or feeling. It was then discovered that different types of nerve ending receptors are widely, but unevenly, distributed in the skin and mucous membranes. For example, the skin of the back possesses relatively few touch and pressure receptors while the fingertips have a great many. The skin of the face has relatively few cold receptors, and the mucous membranes have few heat receptors. The cornea of the eye is sensitive to pain, and when pain sensation is abolished by a local anesthetic, a sensation of touch can be experienced.
There are five kinds of receptors. The most important, those for the sense of touch, are bare nerve endings next to hairs and specialized encapsulated nerve endings called Meissners corpuscles. Cold receptors also have encapsulated nerve endings.
The receptors for pain are naked nerve filaments and are the most numerous; they are also the only kind present in the deeper tissues, although stimulation of these usually causes the pain to be referred to a skin area. Three kinds of pain may be experienced: superficial or cutaneous pain; deep pain from muscles, tendons, joints, and fascia; and visceral pain.
Certain nerve receptors, located in muscles and tendons, are stimulated by changes in tension and pressure and continually inform the brain regarding the position of parts of the body (body sense).
Hunger results from rhythmic contractions of the stomach when it has emptied its contents. Blood sugar levels also influence the feeling of hunger. Habit is another factor; for example, persons who habitually snack in midmorning will feel hunger contractions at normal snacktime. If the snack is not eaten for several days, and adequate food intake continues at mealtime, the hunger contractions at snacktime will diminish. The nervous system also plays a part in controlling 3-39