Connected directly or indirectly with the nasal cavity, of which they
are diverticula, are four pairs of air-sinuses (Sinus paranasales),
viz., maxillary, frontal, sphenopalatine, and ethmoidal.
The =maxillary sinus= (Sinus maxillaris) is the largest. Its external
wall is formed by the maxilla, the lacrimal, and the malar. It is
bounded internally by the maxilla, the inferior turbinal, and the
lateral mass of the ethmoid bone. It extends backward to a transverse
plane in front of the root of the supraorbital process, and its anterior
limit is indicated approximately by a line drawn from the anterior end
of the facial crest to the infraorbital foramen. Its upper boundary
corresponds to a line drawn backward from the supraorbital foramen
parallel to the facial crest. The floor is formed by the alveolar part
of the maxilla; it is very irregular and is crossed by bony plates
running in various directions. The last three cheek teeth project up
into the cavity to an extent which varies with age; they are covered by
a thin plate of bone. The cavity is divided into anterior and posterior
parts by an oblique =septum=. The outer margin of the septum is commonly
about one and a half to two inches (ca. 3.5 to 5 cm.) from the anterior
end of the facial crest; from here it is directed inward, backward, and
upward. The upper part of the septum (formed by the posterior end of the
inferior turbinal bone) is very delicate and usually cribriform.
The position of the septum is quite variable. It is not rare to find
it further forward, and in some cases it is further back than is
stated above. In the recent state, _i. e._, when covered by the mucous
membrane on both surfaces, it is nearly always complete, but in very
exceptional cases there is an opening of variable size in the upper
part.
The =anterior compartment=, often called the =inferior maxillary sinus=,
is partially divided by the infraorbital canal into an external
maxillary part and an internal smaller turbinal part. The latter
communicates with the middle meatus by a very narrow slit situated at
its highest part. The =posterior compartment=, often called the
=superior maxillary sinus=, is also crossed by the infraorbital canal,
internal to which it opens freely into the sphenopalatine sinus. It
communicates dorsally with the frontal sinus through the large oval
=fronto-maxillary opening=, situated at the level of the osseous
lacrimal canal and the corresponding part of the inner wall of the
orbit; the orifice is commonly about one and a half to two inches (ca. 4
to 5 cm.) long and an inch or more (2 to 3 cm.) wide. Just in front of
this, and covered by a thin plate, is the narrow =naso-maxillary
fissure= (Aditus naso-maxillaris), by which the sinus opens into the
posterior part of the middle meatus.
The foregoing statements refer to the arrangement in the average adult
animal. In the foal the cavity (with the exception of its turbinal
part) is largely occupied by the developing teeth. In horses five to
six years of age the maxillary part of the sinus is still filled up to
a large degree by the embedded parts of the teeth. As the teeth are
extruded to compensate the wear, more and more of the cavity becomes
free, until in old age only the short roots project up in the floor,
covered by a layer of bone. Other facts in this connection will be
given in the description of the teeth. In exceptional cases the
posterior part of the inferior turbinal is smaller than usual and
leaves a considerable interval, through which the maxillary sinus
communicates with the nasal cavity.
[Illustration:
FIG. 39.—SKULL OF HORSE, LATERAL VIEW WITHOUT MANDIBLE. THE SINUSES
ARE OPENED UP.
_a_, Posterior part, _b_ and _c_, anterior part of frontal sinus; _d_,
roof of superior meatus; _e_, lateral mass of ethmoid bone; _f_,
_f′_, naso-lacrimal duct, exposed in its posterior part; _g_, _h_,
posterior and anterior compartments of maxillary sinus (also
designated as superior and inferior maxillary sinuses); _i_, septum
between _g_ and _h_; _k_, lower limit of upper thin and partly
membranous portion of septum; _l_, infraorbital canal; _m_, turbinal
part of maxillary sinus; _n_, bullous prominence of inferior
turbinal; _o_, orbit; _p_, infraorbital foramen; _q_, continuation
of infraorbital canal to premaxilla; _r_, limit of maxillary sinus.
(After Ellenberger, in Leisering’s Atlas.)
]
The =frontal sinus= (Sinus concho-frontalis) consists of frontal and
turbinal parts. The frontal part is bounded chiefly by the two plates of
the frontal bone, but its floor is formed in part by the lateral mass of
the ethmoid. It extends forward to a plane through the anterior margins
of the orbits, backward to one through the temporal condyles, and
outward into the root of the supraorbital process. It is separated from
the sinus of the opposite side by a complete =septum=. It is partially
subdivided by a number of bony plates. The turbinal part is situated in
the posterior part of the superior turbinal bone, roofed in by the nasal
and lacrimal bones. It extends forward to a transverse plane about
half-way between the anterior margin of the orbit and the end of the
facial crest. Behind it is in free communication with the frontal part
over the lateral mass of the ethmoid. It is separated from the nasal
cavity by the thin turbinal plate. The frontal and maxillary sinuses
communicate through the large opening described above.
The =sphenopalatine sinus= (Sinus sphenopalatinus) consists of two parts
which communicate under the lateral mass of the ethmoid. The sphenoidal
(posterior) part is excavated in the body of the presphenoid. The
palatine (anterior) part is between the two plates of the perpendicular
part of the palate bone, under the lateral mass of the ethmoid; it
communicates freely with the maxillary sinus. The septum between the
right and left sinuses is not usually median in the sphenoidal part.
In about one-third of the cases (according to Paulli) the sphenoidal
and palatine parts are separated by a transverse septum, and the
sphenoidal part then communicates only with the lower ethmoidal
meatuses.
The term =ethmoidal sinus= is often applied to the cavity of the largest
ethmoturbinal. It communicates with the maxillary sinus.
[Illustration:
FIG. 40.—SKULL OF HORSE, DORSAL VIEW, WITH SINUSES EXPOSED BY REMOVAL
OF THE OUTER PLATE OF BONE.
_1_, Frontal bone; _2_, nasal bone; _3_, lacrimal bone; _4_, maxilla;
_a_, posterior part of frontal sinus; _a′_, middle part of frontal
sinus; _b_, anterior (turbinal) part of frontal sinus; _c_, lateral
mass of ethmoid bone; _d_, roof of superior meatus; _e_,
fronto-maxillary opening; _f_, naso-maxillary opening below plate
which forms the anterior margin of _e_; _g_, _h_, posterior and
anterior compartments of maxillary sinus—often called the superior
and inferior maxillary sinuses; _i_, septum between _g_ and _h_;
_k_, orbit; _l_, point at which superior turbinal bone may be
perforated to obtain drainage into nasal cavity. (After Ellenberger,
in Leisering’s Atlas.)
]