The human mouth is covered by an upper and lower lip. They play an important role in speech (it is part of the vocal apparatus), facial expression, kissing, drinking (especially with a straw), breathing, and smoking. Infants are born with a sucking reflex, by which they instinctively know to suck for nourishment using their lips and jaw. Lips are often adorned with lipstick or lip gloss although in most human cultures this is an affectation for females only. The philtrum is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryo development. When these processes fail to fuse fully, a hare lip and/or cleft palate can result. The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.
According to general etiquette, the mouth is kept closed, especially when chewing.
The mouth starts digestion by physically chewing the food and breaking it down with saliva.
Adapted from: Wikipedia
The cavity of the mouth is placed at the commencement of the digestive tube it is a nearly oval-shaped cavity which consists of two parts: an outer, smaller portion, the vestibule, and an inner, larger part, the mouth cavity proper.
The Vestibule (vestibulum oris) is a slit-like space, bounded externally by the lips and cheeks; internally by the gums and teeth. It communicates with the surface of the body by the rima or orifice of the mouth. Above and below, it is limited by the reflection of the mucous membrane from the lips and cheeks to the gum covering the upper and lower alveolar arch respectively. It receives the secretion from the parotid salivary glands, and communicates, when the jaws are closed, with the mouth cavity proper by an aperture on either side behind the wisdom teeth, and by narrow clefts between opposing teeth.
The Mouth Cavity Proper (cavum oris proprium) is bounded laterally and in front by the alveolar arches with their contained teeth; behind, it communicates with the pharynx by a constricted aperture termed the isthmus faucium. It is roofed in by the hard and soft palates, while the greater part of the floor is formed by the tongue, the remainder by the reflection of the mucous membrane from the sides and under surface of the tongue to the gum lining the inner aspect of the mandible. It receives the secretion from the submaxillary and sublingual salivary glands.
Structure.—The mucous membrane lining the mouth is continuous with the integument at the free margin of the lips, and with the mucous lining of the pharynx behind; it is of a rosepink tinge during life, and very thick where it overlies the hard parts bounding the cavity. It is covered by stratified squamous epithelium.
The Lips (labia oris), the two fleshy folds which surround the rima or orifice of the mouth, are formed externally of integument and internally of mucous membrane, between which are found the Orbicularis oris muscle, the labial vessels, some nerves, areolar tissue, and fat, and numerous small labial glands. The inner surface of each lip is connected in the middle line to the corresponding gum by a fold of mucous membrane, the frenulum—the upper being the larger.
The Labial Glands (glandulœ labiales) are situated between the mucous membrane and the Orbicularis oris, around the orifice of the mouth. They are circular in form, and about the size of small peas; their ducts open by minute orifices upon the mucous membrane. In structure they resemble the salivary glands.
The Cheeks (buccæ) form the sides of the face, and are continuous in front with the lips. They are composed externally of integument; internally of mucous membrane; and between the two of a muscular stratum, besides a large quantity of fat, areolar tissue, vessels, nerves, and buccal glands.
Structure.—The mucous membrane lining the cheek is reflected above and below upon the gums, and is continuous behind with the lining membrane of the soft palate. Opposite the second molar tooth of the maxilla is a papilla, on the summit of which is the aperture of the parotid duct. The principal muscle of the cheek is the Buccinator; but other muscles enter into its formation, viz., the Zygomaticus, Risorius, and Platysma.
The buccal glands are placed between the mucous membrane and Buccinator muscle: they are similar in structure to the labial glands, but smaller. About five, of a larger size than the rest, are placed between the Masseter and Buccinator muscles around the distal extremity of the parotid duct; their ducts open in the mouth opposite the last molar tooth. They are called molar glands.
The Gums (gingivœ) are composed of dense fibrous tissue, closely connected to the periosteum of the alveolar processes, and surrounding the necks of the teeth. They are covered by smooth and vascular mucous membrane, which is remarkable for its limited sensibility. Around the necks of the teeth this membrane presents numerous fine papillæ, and is reflected into the alveoli, where it is continuous with the periosteal membrane lining these cavities.
The Palate (palatum) forms the roof of the mouth; it consists of two portions, the hard palate in front, the soft palate behind.
The Hard Palate (palatum durum) is bounded in front and at the sides by the alveolar arches and gums; behind, it is continuous with the soft palate. It is covered by a dense structure, formed by the periosteum and mucous membrane of the mouth, which are intimately adherent. Along the middle line is a linear raphæ, which ends anteriorly in a small papilla corresponding with the incisive canal. On either side and in front of the raphé the mucous membrane is thick, pale in color, and corrugated; behind, it is thin, smooth, and of a deeper color; it is covered with stratified squamous epithelium, and furnished with numerous palatal glands, which lie between the mucous membrane and the surface of the bone.
The Soft Palate (palatum molle) is a movable fold, suspended from the posterior border of the hard palate, and forming an incomplete septum between the mouth and pharynx. It consists of a fold of mucous membrane enclosing muscular fibers, an aponeurosis, vessels, nerves, adenoid tissue, and mucous glands. When occupying its usual position, i. e., relaxed and pendent, its anterior surface is concave, continuous with the roof of the mouth, and marked by a median raphé. Its posterior surface is convex, and continuous with the mucous membrane covering the floor of the nasal cavities. Its upper border is attached to the posterior margin of the hard palate, and its sides are blended with the pharynx. Its lower border is free. Its lower portion, which hangs like a curtain between the mouth and pharynx is termed the palatine velum.
Hanging from the middle of its lower border is a small, conical, pendulous process, the palatine uvula; and arching lateralward and downward from the base of the uvula on either side are two curved folds of mucous membrane, containing muscular fibers, called the arches or pillars of the fauces.
Adapted from: Yahoo Education