In the adult. The lateral wall is contiguous with the internal carotid artery,

the cavernous blood sinus, first branch of the V cranial nerve, III, IV and VI

cranial nerves (Fig. 24); the roof is related to the frontal lobe, the olfactory

tract, the optic chiasma and the pituitary gland lying in the hypophyseal fossa;

the floor adjoins the pterygoid canal, roof of the nasal cavity, nasopharynx;

while the medial wall is a septum separating it from its neighbour. Anterior

wall is contiguous with ethmoidal cells. Behind the posterior wall lies the

posterior cranial fossa. The ostium is placed high up in the cavity of the

sinus.

BASIC PHYSIOLOGY

The nose is both a sense organ and a respiratory organ. In addition, the nose

performs an important function for the entire body by providing both physical

and immunologic protection from the environment. It is also important in the

formation of speech sounds.

The Nose as an Olfactory Organ

The human sense of smell is poorly developed compared to most mammals and

insects. Despite that, it is still very sensitive in the human and is almost

indispensable for the individual. For example, taste is only partially a

function of the taste buds since these can only recognize the qualities of

sweet, sour, salty, and bitter. All other sensory impressions caused by food

such as aroma and bouquet are mediated by olfaction. This gustatory olfaction is

due to the fact that the olfactory substances of food or drink pass through the

olfactory cleft during expiration while eating or drinking. The sense of smell

can stimulate appetite but can also depress it. It also provides warning of

rotten or poisonous foods and also of toxic substances, e.g., gas. The sense of

smell is particularly important in the field of psychology: Marked affects may

be induced or inhibited by smells. It should also be remembered that a good

sense of smell is essential for those in certain occupations, e.g., cooks,

confectioners, wine, coffee, and tea merchants, perfumers, tobacco blenders, and

chemists. Finally, the physician needs a "clinical nose" for making his

diagnosis.

The olfactory area of the nose is relatively small. It contains the olfactory

cells, i.e., the bipolar nerve cells, which are to be regarded as the sensory

cells and first-order neurons. They are collected into about 20 fibers in the

olfactory nerves which run to the primary olfactory center of the olfactory

bulb.

From here the neurons of the bulb run via the olfactory tract to the secondary

olfactory center. The tertiary cortical olfactory field lies in the dentate and

semilunate gyri.

The mode of action of the scent molecules on the olfactory cells is not known

with certainty. There are numerous current theories of the mechanism of action,

including: emission of scent corpuscles, selective absorption, specific

receptors on the sensory cells, enzymatic control, molecular vibrations,

electrobiologic processes such as changes in cell membrane potential, etc.

It is certain that only volatile substances can be smelled by humans. These

substances must be soluble in water and lipids. Only a few molecules suffice to

stimulate the sense of smell. 10-15 molecules per ml of air are sufficient

stimulation on average to exceed the threshold.

It is said that there are about 30000 different olfactory substances in the

atmosphere; of these, humans can perceive about 10000 and are able to

distinguish among 200.

The sense of smell, like other senses, demonstrates the phenomenon of

adaptation. The sensitivity of the olfactory organ depends also on hunger:

several olfactory factors can be smelled better if the subject is very hungry

than shortly after eating, a very useful physiologic regulation.

Anosmia and hyposmia may be caused by obliteration of the olfactory cleft

(polyps, etc.), causing respiratory anosmia. Inability of the olfactory

substances in food and drink to pass from the mouth and throat to the olfactory

epithelium of the nose because of obstruction of the nasal cavity or the choana

is described as gustatory anosmia. Central anosmia is caused by a disorder of

the central nervous parts of the olfactory system in the presence of a patent

airway. Causes include: traumatic rupture of the olfactory nerve, cerebral

contusion, and cerebral diseases. Essential anosmia is due to local damage to

the olfactory epithelium, e.g., due to influenza, with an open olfactory cleft.

The Nose as a Respiratory Organ

In the human the only physiologic respiratory pathway is via the nose. Mouth

breathing is unphysiologic and is only brought into play in an emergency to

supplement nasal respiration. The physiology of the airstream through the normal

nose in inspiration and expiration may be summarized as follows:

The average ventilation through a normal nose in physiologic breathing is 6

l/min, and 50 to 70 l/min in maximal ventilation.

The internal nasal valve or limen nasi is the most narrow point in the normal

nose. It thus acts like a nozzle, and the speed of the airstream is very high at

this point.

The nasal cavity between the valve and the head of the turbinates acts as a

diffusor, i.e., it slows the air current and increases turbulence. The central

part of the nasal cavity with its turbinates and meatus is the most important

part for nasal respiration. The column of air consists of a laminar and a

turbulent stream. The proportion between laminar and turbulent flow considerably

influences the function and condition of the nasal mucosa.

The airstream passes in the reverse direction through the nasal cavities on

expiration. The expiratory airstream demonstrates considerably less turbulence

in the central part of the nose, and thus offers less opportunity for heat and

metabolic exchange between the airstream and the nasal wall than on inspiration.

The nasal mucosa can thus recover during the expiratory phase. Inspiration

through the nose followed by expiration through the mouth leads rapidly to

drying of the nasal mucosa.

Complete exclusion of the nose from breathing leads in the long term to

deep-seated mucosal changes. Mechanical obstruction within the nose, e.g., due

to septal deviation, hypertrophy of the turbinates, cicatricial stenoses, etc.,

can lead to mouth breathing and its damaging consequences and can also cause

mucosal diseases of the nose and nasal sinuses.

The nasal patency can be influenced by many different factors, including

temperature and humidity of the surrounding air, the position of the body,

bodily activity, changes of body temperature, the influence of cold on different

parts of the body, e.g., the feet, hyperventilation, and psychological stimuli.

The state of the pulmonary function, of the heart, and of the circulation,

endocrinologic disorders such as pregnancy, hyper- or hypofunction of the

thyroid gland, and some local, oral, or parenteral drugs may have considerable

influence on the patency of the nose.

Protective function of the Nose

During normal nasal respiration, the inspired air is warmed, moistened, and

purified during its passage through the nose.

The warming of the inspired air through the nose is very effective, and the

constancy of the temperature in the lower airways is remarkably stable. The

nasal mucosa humidifies and warms the air. The temperature in the nasopharynx

during normal (exclusively nasal) respiration is constant at 31° to 34°C

independent of the external temperature. The heat output of the nose increases

as the external temperature falls so that the lower airways and the lungs can

function at the correct physiologic temperature.

The optimal relative humidity of room air for subjective well-being and function

of the nasal mucosa lies between 50% and 60%. The water vapor saturation of the

inspired air in the nasopharynx lies between 80% and 85%, and in the lower

airway is fairly constant between 95% and 100%, independent of the relative

humidity of the environmental air. The water vapor secreted by the entire

respiratory tract per 1000 liters of air can reach 30 g. Most of this is

supplied by the nose. On the other hand, the mucosal blanket renders the nasal

mucosa watertight and prevents release of too much water into the air, which

would cause drying of the mucosa.

The cleaning function of the nose includes: first, cleaning of the inspired air

from foreign bodies, bacteria, dust, etc., and second, cleaning of the nose

itself. About 85% of particles larger than 4,5 mm are filtered out by the nose,

but only about 5% of particles less than 1 mm in size are removed.

Foreign bodies entering the nose come into contact with the moist mucosal

surface and the mucosal blanket, which continually carries away the foreign

bodies.

The Nasal Mucosa as a Protective Organ

In addition to warming, humidifying, and cleaning the inspired air, the nose

also has a protective function consisting of a highly differentiated, efficient,

and polyvalent resistance potential against environmental influences on the

body. A basic element of this defensive system is the mucociliary apparatus.

This is the functional combination of the secretory film and the cilia of the

respiratory epithelium by which the colloidal secretory film is transported

continuously from the nasal introitus toward the choana. A foreign body is

carried from the head of the inferior turbinate to the choana in about 10 to 20

min. The efficiency of this cleansing system depends on several factors such as

pH, temperature, condition of the colloids, humidity, width of the nose, toxic

gases, etc. Disturbances in the composition or in the physical characteristics

of the mucosal blanket or of the ciliary activity can have marked influences on

the physiology of the nasal cavity.

The nasal mucosa protects the entire body by making contact with and providing

resistance against animate and inanimate foreign material in the environment.

Two defence zones can be distinguished in the nasal mucosa: first, the mucosal

blanket and the epithelium, and second, the vascular connective tissue of the

lamina propria.

Resistance factors of the first defensive zone include: (1) physical cleaning by

the mucociliary apparatus; (2) nonspecific protective factors in the secretions

such as lysozymes, interferon, secretory protease inhibitors, complement system,

and secretory glucosidases; and (3) specific protective factors such as