_1_, Superior turbinal fold; _2_, alar fold, containing cartilaginous
prolongation of inferior turbinal; _3_, arrow pointing to
naso-maxillary fissure; _4_, cranial cavity; _5_, sphenoidal sinus;
_6_, hamulus of pterygoid bone. The olfactory mucous membrane is
shaded. Dotted lines indicate anterior limit of uncoiled parts of
turbinal bones, which inclose parts of frontal and maxillary
sinuses.
]
The =middle meatus= (Meatus nasi medius) is between the two turbinal
bones. It is somewhat larger than the superior meatus, and does not
extend back so far; it ends near the great ethmoturbinal and the
ethmoidal meatuses. In its posterior part is found the extremely narrow,
slit-like interval by which the maxillary sinus communicates with the
nasal cavity—the =naso-maxillary fissure= (Aditus naso-maxillaris). The
fissure is not visible from the nasal side, being concealed by the
overhanging superior turbinal. A fine flexible probe, passed outward and
somewhat backward between the turbinal bones, enters the maxillary
sinus; if introduced a little further in the same direction, it usually
passes through the orifice of communication between the maxillary and
the frontal sinus and enters the latter. A small part of the fissure
usually brings the anterior division of the maxillary sinus into
communication with the nasal cavity. The spaces inclosed by the coiled
portions of the turbinals also open into the middle meatus. This passage
may be characterized as the sinus-meatus, but it also conducts air to
the olfactory region.
[Illustration:
FIG. 348.—CROSS-SECTION OF HEAD OF HORSE. THE SECTION PASSES THROUGH
THE INTERNAL CANTHI.
_1_, Mylo-hyoideus; _2_, stylo-glossus; _3_, hyo-glossus; _4_,
genio-glossus; _5_, lingual process of hyoid bone. An arrow points
to the naso-maxillary fissure.
]
The =inferior meatus= (Meatus nasi ventralis) is situated between the
inferior turbinal and the floor of the cavity. It is much larger than
the other two, and is the direct passage between the nostrils and the
pharynx.
The =common meatus= (Meatus nasi communis) is situated between the
septum and the turbinals, and is continuous externally with the other
meatuses. It is very narrow dorsally, but widens ventrally.
The lateral masses of the ethmoid bone project forward into the
posterior part (fundus) of the nasal cavity (Fig. 347). Between the
ethmoturbinals of which each mass is composed there are three principal
and numerous small passages, the =ethmoidal meatuses= (Meatus
ethmoidales).
The =posterior nares= (Choanæ) are two elliptical orifices by which the
nasal cavity and pharynx communicate. They are in the same plane as the
floor of the nasal cavity, and are separated from each other by the
vomer. They are, taken together, about two inches (ca. 5 cm.) wide and
three to four inches (ca. 8 to 10 cm.) long.
The =nasal mucous membrane= (Membrana mucosa nasi) is thick, highly
vascular, and is, in general, firmly attached to the underlying
periosteum and perichondrium. It is continuous in front with the skin
which lines the nostrils, and behind with the mucous membrane of the
pharynx. It is also continuous at the naso-maxillary fissure with the
very thin and only slightly vascular mucous membrane which lines the
sinuses. In the anterior part of the cavity it forms prominent thick
folds on the lateral wall, which extend from the turbinal bones to the
nostril. There are usually two =superior turbinal folds= which unite
anteriorly. The upper one incloses a thin plate of cartilage which is
continuous with the superior turbinal bone. The =inferior turbinal fold=
is curved, and incloses an ᔕ-shaped cartilaginous plate which prolongs
the inferior turbinal bone; this fold is continuous with the alar fold
of the nostril, and forms with it the upper margin of the entrance from
the true nostril to the nasal cavity. Below this there is a rounded
ridge produced by the nasal process of the premaxilla. The mucous
membrane of the greater part of the cavity (Regio respiratoria) is red
in color, and is covered with a stratified ciliated epithelium. It
contains numerous acinous =nasal glands= (Glandulæ nasales). The
submucosa contains rich venous plexuses which form in certain situations
a sort of cavernous tissue (Plexus cavernosus), composed of several
strata of freely anastomosing veins, between which there are unstriped
muscle-fibers. This arrangement is most marked in the turbinal folds, on
the lower part of the inferior turbinal bone, and the lower part of the
septum. On the posterior part of the lateral masses of the ethmoid and
the adjacent part of the superior turbinal and the septum (Regio
olfactoria) the mucous membrane is brownish-yellow in color and thicker;
it contains the olfactory nerve-endings in a special non-ciliated
epithelium. In it are numerous =olfactory glands= (Glandulæ olfactoriæ);
these are long, tubular, and often branched.
The =vomero-nasal organ= of Jacobson (Organon vomeronasale) is situated
on the floor of the nasal cavity, on either side of the ventral border
of the septum (Fig. 345.) It consists of a tubular cartilage lined with
mucous membrane, to which fibers of the olfactory nerve may be traced.
Its anterior part communicates with the nasal cavity by a slit-like
orifice.
The =paranasal sinuses= are described in the Osteology.