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90 ANATOMY.
walls are much thicker than in the adult. Its capacity varies in dif-
ferent subjects and in the opposite bones of the same subject, ranging
from one drachm to one ounce fluid measure, the average being about
three drachms. It is somewhat larger in the male than in the female.
The floor of the sinus is marked by irregular eminences correspond-
ing to the roots of the molar teeth. Somedmes it is punctured by the
roots of these teeth, which may extend into the sinus. The walls of
the sinus often support thin plates of bone which subdivide it into
small compartments. A knowledge of this fact is of importance in
operating on tumors and abscesses in this location, as the drill may
simply penetrate one of these distinct compartments, misleading the
operator as to the extent of the sinus or of the disease.
The sinus opens into the middle meatus of the nose by an orifice
of variable size situated at the base of the pyramid. This orifice
is partly closed by the uncinate process of the ethmoid, the vertical
plate of the palate, the inferior turbinated, and the lachrymal bones,
also by soft tissue, so that in the recent state it is about the size of an
ordinary lead-pencil. This small opening is situated near the upper
part of the sinus, and does not, when the head is perpendicular, afford
a ready outlet to fluids collected within the chamber.
The mucous membrane lining the sinus is ciliated, and is continuous
through this quill-like opening with the membrane lining the nasal
cavity. It is, however, less vascular than the nasal mucous membrane.
The sinus may be encroached upon by any of the teeth situated in
the superior maxilla, but those that most frequently protrude into it are
the roots of the first and second molars.
With the exception of the alveolar border the walls of the sinus are
quite thin. These walls are four in number—one extending toward the
orbit ; another toward the nose and roof of the mouth, including a
portion of the palatal process ; a third toward the facial and zygomatic
surfaces of the bone; and a fourth is formed by the alveolar border.
Morbid growths within the sinus will more readily cause either of the
first three walls to project or bulge than the alveolar process.
The ])osterior wall, or that toward the zygomatic surface of the bone,
is marked by the j)osterior dental canals, through which the posterior
dental vessels and nerves are conducted to the teeth. The anterior M^all
is in like manner grooved by the anterior dental or incisor vessels
arid nerves.
Palate Bone.

The palate bones, two in number, arc Mcdged between the superior
maxillary bones and the pterygoid ])lates of the sphenoid bone at the
back part of the nasal chambers. Tluy assist in forming the bound-
aries of the (H'bital, nasal, and oral cavities, the spheno-maxillary, the
spheno-palatine, and the ])terygoid fossa, the s})heno-maxillary fissure,
the ])()st('rior etlimoidal cells, and the maxillary sinus.
The jialate bone is com])(>sed of thin,- delicate, and compact tissue.
Its general iorm is that of tlie letter L. It is composed of two plates,
a horizontal and v'ertical ; and three processes, a pyramidal, orbital, and
sphenoid.