The mouth is the first part of the gastrointestinal tract and is
equipped with several structures that begin the first processes of
digestion.[3] These include salivary glands, teeth and the tongue. The
mouth consists of two regions, the vestibule and the oral cavity
proper. The vestibule is the area between the teeth, lips and
cheeks,[4] and the rest is the oral cavity proper. Most of the oral
cavity is lined with oral mucosa, a mucous membrane that produces a
lubricating mucus, of which only a small amount is needed. Mucous
membranes vary in structure in the different regions of the body but
they all produce a lubricating mucus, which is either secreted by
surface cells or more usually by underlying glands. The mucous
membrane in the mouth continues as the thin mucosa which lines the
bases of the teeth. The main component of mucus is a glycoprotein
called mucin and the type secreted varies according to the region
involved. Mucin is viscous, clear, and clinging. Underlying the mucous
membrane in the mouth is a thin layer of smooth muscle tissue and the
loose connection to the membrane gives it its great elasticity.[5] It
covers the cheeks, inner surfaces of the lips, and floor of the
mouth.[6]:1186

The roof of the mouth is termed the palate and it separates the oral
cavity from the nasal cavity. The palate is hard at the front of the
mouth since the overlying mucosa is covering a plate of bone; it is
softer and more pliable at the back being made of muscle and
connective tissue, and it can move to swallow food and liquids. The
soft palate ends at the uvula.[7] The surface of the hard palate
allows for the pressure needed in eating food, to leave the nasal
passage clear.[8] The lips are the mouth's front boundary and the
fauces (the passageway between the tonsils, also called the
throat),[6]:686 mark its posterior boundary. At either side of the
soft palate are the palatoglossus muscles which also reach into
regions of the tongue. These muscles raise the back of the tongue and
also close both sides of the fauces to enable food to be
swallowed.[6]:1208 Mucus helps in the mastication of food in its
ability to soften and collect the food in the formation of the bolus.