Mouth

Ingested food is changed into a bolus by the mechanical action of
teeth and by the chemical activity of saliva. The bolus is swallowed
in the process of deglutition.

The functions of the mouth and associated structures are to form a
receptacle for food, to initiate digestion through mastication, to
swallow food, and to form words in speech. The mouth can also
assist the respiratory system in breathing air. The pharynx, which
is posterior to the mouth, serves as a common passageway for both
the respiratory and digestive systems. Both the mouth and pharynx
are lined with nonkeratinized stratified squamous epithelium,
which is constantly moistened by the secretion of saliva.
The mouth, also known as the oral cavity (fig. 18.5), is
formed by the cheeks, lips, hard palate and soft palate. The
vestibule of the oral cavity is the depression between the cheeks
and lips externally and the gums and teeth internally (fig. 18.6).
The opening of the oral cavity is referred to as the oral orifice,
and the opening between the oral cavity and the pharynx is
called the fauces (faw/se¯z).

Cheeks, Lips, and Palate

The cheeks form the lateral walls of the oral cavity. They consist
of outer layers of skin, subcutaneous fat, facial muscles that assist
in manipulating food in the oral cavity, and inner linings of
moistened stratified squamous epithelium. The anterior portion
of the cheeks terminates in the superior and inferior lips that surround
the oral orifice.

The lips are fleshy, highly mobile organs whose principal
function in humans is associated with speech. Lips also serve for
suckling, manipulating food, and keeping food between the upper
and lower teeth. Each lip is attached from its inner surface to the
gum by a midline fold of mucous membrane called the labial
frenulum (fren'yu˘-lum) (fig. 18.5). The lips are formed from the
orbicularis oris muscle and associated connective tissue, and are
covered with soft, pliable skin. Between the outer skin and the
mucous membrane of the oral cavity is a transition zone called
the vermilion. Lips are red to reddish brown because of blood
vessels close to the surface. The numerous sensory receptors in
the lips aid in determining the temperature and texture of food.



The palate, which forms the roof of the oral cavity, consists
of the bony hard palate anteriorly and the soft palate posteriorly
(figs. 18.5). The hard palate, formed by the
palatine processes of the maxillae and the horizontal plates of
the palatine bones, is covered with a mucous membrane.
Transverse palatine folds, or palatal rugae (roo'je), are located
along the mucous membrane of the hard palate. These structures
serve as friction ridges against which the tongue is
placed during swallowing. The soft palate is a muscular arch
covered with mucous membrane and is continuous with the
hard palate anteriorly. Suspended from the middle lower border
of the soft palate is a cone-shaped projection called the
palatine uvula (yoo'vyu˘-la˘). During swallowing, the soft palate
and palatine uvula are drawn upward, closing the nasopharynx
and preventing food and fluid from entering the nasal
cavity.

Two muscular folds extend downward from both sides of
the base of the palatine uvula (figs. 18.5 and 18.6). The anterior
fold is called the glossopalatine arch, and the posterior fold is
the pharyngopalatine arch. Between these two arches is the
palatine tonsil.