8 THE NOSE AND PARANASAL SINUSES
over the posterior end of the inferior concha. Little or no air seems to travel
through the inferior meatus. Expired air follows a similar pathway in reverse,
but some of it forms eddies around the middle concha. There is a orhythmic
alternation in the use of either nasal cavity, and both sides are seldom used
equally at the same time.
During its inspiratory passage through the nasal cavity the air is filtered,
warmed and moistened so that, whatever the state of the outside air, it is
delivered through a normal nose to the lungs in a stable condition as regards
warmth and humidity. If a tracheostomy has been performed, thus bypassing
the nasal cavity, the air entering the bronchioles is too cold and too dry,
unless the patient is nursed in an appropriate atmosphere, and the bronchi
respond to this stimulus by producing an excessive secretion of mucus.
Similar, if less dramatic, changes occur in children who adopt mouth
Filtration is accomplished by the vibrissae in the nasal vestibule which
enmesh larger particles of fluff and dust, and by the nasal mucous blanket
which covers the mucous membrane and is constantly propelled posteriorly by
the cilia of the lining membrane. This mucous blanket is adhesive, and
bacteria and particles of dust adhere to it. It is also, to some extent, bactericidal
by virtue of its lysozyme content. Warming of the air is achieved by the
vascular conchae with their submucous blood spaces, and the air is moistened
by absorption of water content from the seromucinous gland secretions.
A nose depends for its health upon the mucous glands and the ciliated
epithelium which keeps the mucus in constant movement. The cilia are robust
enough to function even in infections, but their action is slowed by oily drops
or sprays, and is destroyed by drying. For this reason centrally heated premises
should have the air humidified, and for this reason also surgeons should
respect the integrity of the surface epithelium during operative procedures.
Olfactory. The olfactory sense is less well developed in man than in some of
the lower animals, but it is still sufficient to allow human beings to perceive
odours in extreme dilution. The acuity varies greatly between individuals.
The direction of the air current ensures that airborne odorous substances
reach the olfactory area, and this is increased by forced inspiration or
sniffing. Olfactory cells are stimulated by these substances, but they may also
be stimulated by the blood stream. In order that an odour may be perceived a
sufficient volume of the air containing the odour must reach the olfactory
area, and the olfactory mechanism must be unimpaired. Thus, nasal conges-
tion or obstruction, as, for example, during the common cold or in the
presence of nasal polypi, diminishes olfactory acuity. Similarly, the effects of
toxins, such as virus infections and certain poisons, may reduce the efficiency
of the olfactory pathway, while a fracture through the cribriform plate of the
ethmoid will destroy it. The acuity of smell may be estimated by the applica-
tion of varying strengths of different substances, such as volatile oils, to the
nostrils. The olfactory sense is allied to the sense of taste which is also affected
if the former is impaired.
THE PARANASAL SINUSES
The paranasal sinuses, arranged in pairs and in relation to each nasal cavity,
comprise two groups, anterior and posterior. The former includes the