Info

Anterior fold

Light reflex

Handle of the malleus

Figure 11-6 Cross-sectional view through the cochlea.

Figure 11-5 Landmarks of the left tympanic membrane.

The facial nerve passes through the middle ear and provides, in addition to the nerve to the stapedius muscle, the chorda tympani nerve. The chorda tympani travels through the middle ear between the incus and the malleus and exits near the temporomandibular joint. It carries taste sensation from the anterior two thirds of the tongue.

The inner ear is the end-organ for hearing and equilibrium. It is situated in the petrous portion of the temporal bone and consists of the three semicircular canals, the vestibule, and the cochlea. Each of these structures is made up of three parts: the osseous labyrinth, the membranous labyrinth, and the space between. The osseous labyrinth is the outer bone casing. The inner membranous labyrinth is within the osseous labyrinth and contains a fluid called endo-lymph and the sensory structures. The space between these two labyrinths is filled with another fluid, called perilymph. A cross section through this area is illustrated in Figure 11-6.

The three semicircular canals are directed posteriorly, superiorly, and horizontally. Each canal has a dilated end, the ampulla, which is the sensory end-organ for balance.

The cochlea is a snail shell-shaped structure composed of 24 turns. Within its membranous labyrinth is the end-organ for hearing. The acoustic, or eighth cranial, nerve consists of two parts: the vestibular and cochlear divisions. These connect to the semicircular canals and cochlea, respectively. They join and pass through the internal auditory meatus to the brain stem.

Sound waves stimulate the afferent fibers either by bone conduction or by air conduction. Bone conduction is directly through the bones of the skull. Air conduction is through the external auditory canal, tympanic membrane, and ossicles to the oval window. Most hearing is mediated by air conduction.

Sound waves set up vibrations that enter the external canal and are transmitted to the ossicles, which vibrate. This vibration causes an inward motion of the footplate of the stapes and deforms the oval window. Waves are created in the perilymphatic fluid of the labyrinth. These fluid motion changes are transmitted in a wavelike manner to the endolymphatic fluid, which causes distortion of the hair cells of the organ of Corti. These hair cells convert the mechanical force into an electrochemical signal that is propagated down the acoustic nerve and is ultimately interpreted as sound. It has been estimated that there are more than 30,000 of these afferent hair fibers, which constitute the auditory division. After many synapses, the impulse reaches the temporal cortex, where the appreciation of the sound occurs. A cross section through the cochlear duct is shown in Figure 11-7.

The sense of balance is achieved by visual, vestibular, and proprioceptive* senses. The loss of one of these senses frequently goes unnoticed. The vestibular apparatus appears to be the most important. Motion within the endolymphatic fluid stimulates the hair cells in the ampulla of the semicircular canals. Electrical impulses are transmitted to the vestibular portion of the eighth cranial nerve. Synapses occur in the vestibular and oculomotor nuclei,

*Sensory stimulation from within the tissues of the body with regard to their movement or position.

which send efferent fibers to the extraocular and skeletal muscles. This produces a deviation of the eyes with rapid compensatory motions to maintain gaze and increased tone in the skeletal muscles.

Any alteration in the endolymphatic mechanism may affect the control of the eyes. Nystagmus is an involuntary, rapid back-and-forth eye motion, which can be horizontal, vertical, rotatory, or mixed. The direction of the nystagmus is determined by the direction of the quick component. Abnormalities of the labyrinth tend to produce horizontal nystagmus; brain-stem disorders often produce vertical nystagmus;and retinal lesions may produce ocular nystagmus, which is slow and produces an irregular, searching quality to the eyes.

The Nose

The external nasal skeleton consists of the nasal bones, part of the maxilla, and the cartilage. The upper third of the skeleton is composed of nasal bones, which articulate with the maxilla and frontal bones. The lower two thirds is made of cartilage.

The internal portion of the nose consists of two cavities divided by the nasal septum, which forms the medial wall of the nasal cavity. Projecting from the lateral wall are three turbinates, or conchae. The inferior turbinate is the largest and contains semierectile tissue. Inferior to each turbinate are openings to the paranasal sinuses; each opening is known as a meatus. Each meatus is named for the turbinate above it. The nasolacrimal duct empties into the inferior meatus. The middle meatus, below the middle turbinate, contains the openings of the frontal, maxillary, and anterior ethmoid sinuses. The posterior ethmoid sinus drains into the superior meatus. The olfactory region is located high in the nose between the nasal septum and the superior turbinate. Figure 11-8 illustrates the lateral wall of the nose.

The blood supply to the nose is derived from the internal and external carotid arteries. The turbinates are vascular and contain large vascular spaces. The blood vessels of the anterior nasal septum meet at an area about 1 inch (2.54 cm) from the mucocutaneous junction, known as Little's area. This is the area usually responsible for epistaxis, or nosebleed. The blood vessels are under autonomic nervous system control. If there is an excess of sympathetic stimulation, the blood vessels constrict, and the vascular spaces in the turbinates shrink. If there is an increase in parasympathetic tone, the blood pools in the turbinates, resulting in their swelling, obstruction to airflow, and elaboration of a watery discharge.

The nerve supply to the internal area of the nose is from branches of the trigeminal nerve. The olfactory epithelium is supplied by the olfactory, or first cranial, nerve. Moist air with the dissolved odorous particles acts as a stimulus. The nerve fibers from this area pierce the cribriform plate to the olfactory bulb in the brain. In humans, the olfactory receptors' ability to discern a stimulus diminishes rapidly with exposure to the stimulus.

The main functions of the nose are to provide the following:

An airway Olfaction

Humidification of inspired air Warming of inspired air Filtering of inspired air

The inspired air flows above and below the middle turbinate. This produces eddy currents that serve to protect the olfactory epithelium in the superior portion of the nose. The nasal mucosa produces mucus, which increases the relative humidity to nearly 100%. This prevents the epithelium from drying out and helps prevent possible infection. The air, by its circulation around the conchae, is warmed to nearly body temperature by the time it enters the nasopharynx. The mucus and the nasal hairs, or vibrissae, prevent particulate matter from entering the distal respiratory tract. The mucous blanket is swept posteriorly by the cilia and is swallowed. The mucus also contains immunoglobulins and enzymes, which serve as a line of defense.

The four paranasal sinuses of the head—the maxillary, the ethmoid, the frontal, and the sphenoid—are air-filled cavities lined with mucous membranes. The maxillary sinus is the largest and is bounded by the eye, the cheek, the nasal cavity, and the hard palate. The ethmoid sinuses are multiple and are present in the ethmoid bone, which lies medial to the orbit and extends to the pituitary fossa. The frontal sinus is located above the ethmoid sinuses and is bounded by the forehead, the orbit, and the anterior cranial fossa. Behind the ethmoid sinuses is the sphenoid sinus. The paranasal sinuses have no known functions. The maxillary, frontal, and ethmoid sinuses and their connections to the nose are illustrated in Figure 11-9.

Superior turbinate Superior meatus

Middle turbinate

Middle meatus Inferior turbinate

Inferior meatus

Nasal bone Cartilage

Nasal septum Maxilla

Figure 11-8 Lateral wall of the nose.

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