The =nasal cavity= (Cavum nasi), the first segment of the respiratory
tract, is a long, somewhat cylindrical passage, inclosed by all the
facial bones except the mandible and hyoid. It is separated from the
mouth ventrally by the palate. It opens externally at the nostrils, and
communicates posteriorly with the pharynx through the posterior nares.
The =nostrils= or =anterior nares= (Nares) are somewhat oval in outline,
and are placed obliquely, so that they are closer together below than
above. They are bounded by two =alæ= or =wings= (Alæ nasi), which meet
above and below, forming the =commissures=. The outer ala is concave;
the inner one is convex above, concave below. The upper commissure is
narrow, the lower one wide and rounded. If the finger is passed into the
nostril at the upper commissure, it enters the so-called =false
nostril=, which leads backward to a blind cutaneous pouch. This
cul-de-sac, the =diverticulum nasi=, extends backward to the angle of
junction of the nasal bone and the nasal process of the premaxilla. The
posterior part of the diverticulum is blind, but the false nostril
communicates below and internally with the vestibule of the nasal
cavity. The external opening of the =naso-lacrimal duct= is seen when
the nostril is dilated; it is situated on the floor of the vestibule,
perforating the skin close to its junction with the mucous membrane,
about two inches (ca. 5 cm.) from the lower commissure. (It is not rare
to find one or two accessory orifices further back.)
=Structure.=—The =skin= around the nostrils presents long tactile hairs
as well as the ordinary ones. It is continued around the alæ and lines
the vestibule. In the posterior part of the diverticulum the skin is
thin and usually black, and is covered with exceedingly fine hairs; it
is provided with numerous sebaceous glands. The inner wing is supported
by the =alar cartilage= (Cartilago alaris), which is shaped somewhat
like a comma, the convex margin being direct medially. The cartilages
are attached by fibrous tissue to the extremity of the septal cartilage.
Each consists of an upper, quadrilateral curved plate, the =lamina=, and
a narrow =cornu= which curves downward and outward, supporting the inner
wing and the lower commissure, but not entering into the formation of
the outer wing. The lamina causes the projection of the upper part of
the internal wing, from which the thick =alar fold= (Plica alaris)
passes backward along the external wall of the nasal cavity to cover the
cartilaginous prolongation of the inferior turbinal bone. This fold
separates the true from the false nostril. The extremity of the cornu
usually causes a slight projection of the skin a short distance behind
and below the lower commissure.
[Illustration: