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BONES. 137

The slit-like openings in the anterior portion of the cribriform plate
communicate with the brain-case, and transmit the nasal nerve and the
vessels which accompany it.
The Openings into Air-cells.—In the dried skull there are generally
two openings into the maxillary sinus, but in the recent state there is
not often more than one. It is situated about the centre of the middle
meatus, and permits the passage of fluid from the antrum into the nasal
chambers.
The Infundihulum extends from the superior portion of the middle
meatus anteriorly to the outer side of the middle turbinated bone, unit-
ing this meatus with the frontal sinuses and the anterior ethmoidal
it allows the fluid from these openings to descend into the nasal
cells ;
chambers.
The other openings into the remaining air-cells have already been
fully described under the headings of the bones.

The Cavity of the Mouth.
The Cavity of the Mouth is situated between the two superior maxil-
lary bones above and the inferior maxillary bone and their attached
muscles. When the jaws are closed this cavity is paraboloid in shape,
opening behind and below. In the recent state the inferior opening is
closed by the tongue and mylo-hyoid muscle.
The Roof of the Mouth (Fig. QQ), which is formed by the hard palate,
is generally arched in front and flattened behind. It is composed of
the palatal processes of the
Fig. 66.
two superior maxillary and
palate bones.
The Posterior Border is
free, thin, and divided into
two portions by the posterior
nasal spine. On each side
of this spine the border is
concave, and terminates later-
ally in the pyramidal process
of the palate bone and the
hamular process of the sphe-
noid bone.
Situated within the palato-
maxillary suture, just inter-
nal to the tuberosity of the
superior maxilla, are the pos-
Roof of the Mouth.
terior and accessory palatine
canals for the transmission of the posterior palatine nerves and vessels.
The surface of the roof of the mouth is perforated by numerous
small foramina for the transmission of nutrient vessels to the body of
the bone, pitted for the lodgment of the mucous glands, and grooved
longitudinally for the accommodation of vessels.
The Floor of the 3Iouth.—The circumference of the floor of the
mouth is formed by the mylo-hyoid ridge. This ridge gives attach-