The lips [labia oris] form the anterior wall of the mouth cavity. The lower lip [labium inferius] is marked off from the chin by the sulcus mentolabialis. The upper lip [labium superius] extends upward to the nose medially and the sulcus nasolabialis laterally. The philtrum is a median groove on the upper lip extending from the septum of the nose above to the labial tubercle [tuberculum labii superioris] below, at the middle of the rima oris. On each side of the rima oris the upper and the lower lips are continuous at the angle of the mouth [angulus oris], which is usually opposite the first premolar teeth. Laterally, the lips are continuous with the cheeks [buccse], which form the lateral walls of the mouth cavity.
In structure, the lips consist essentially in a middle layer of cross-striated muscle (orbicularis oris) covered externally by skin which is continuous through the rima oris with the mucosa forming the inner layer of the lips. The mucosa lines the vestibulum oris and is reflected upon the gums above and below. In the median line above and below, there extends fi-om the lip to the gum a small fold of the mucosa [frenulum labii superioris vel inferioris]. The structure of the cheeks is similar to that of the lips but somewhat more complicated.
The muscular basis of the cheek is the buccinator muscle. External to this is a thick layer of fat [corpus adiposum buccae] covered partly by the dermal muscles (platysma, zygomaticus, etc.) and lastly the skin. Internally the cheek is lined by the mucosa, continuous with that of the cheeks. The parotid duct opens into the vestibule opposite the second upper molar tooth.
Glands of the lips and cheeks
The skin of the lips and cheeks is well supplied with the usual sudoriparous and sebaceous glands. The mucosa likewise presents two kinds of glands, the sebaceous and the mucous glands. The sebaceous glands are relatively few in number and variable, being present in about 30 per cent, of cases in the adult (Stieda). They are similar in structure to those of the skin (though not associated with hair follicles), and when present are visible as small yellowish bodies in the mucosa. They occur chiefly near the free margins of the lips and along the cheek opposite the teeth.
The mucous glands are much more numerous and constantly present. They are all of the racemose type. They are variable but small in size, and closely packed together in the submucosa of the lips [glandulae labiales], where they may easily be felt. Those of the cheeks [gl. buccales] are less numerous. A few of them especially in the region of the molar teeth [gl. molares], are placed outside the buccinator. The ducts of the molar glands pierce this muscle near the parotid duct to open on the surface of the mucosa.
Vessels and nerves of the lips and cheeks
The mucosa of the lips and cheeks has a characteristic reddish hue, on account of the numerous blood-vessels which are visible through the thick but transparent stratified squamous epithelium. The numerous papillae of the lamina propria are highly vascular. The blood-supply of the lips and cheeks is derived chiefly from the labial (coronary) and buccal arteries. The rich nerve-supply (sensory) is from the infra-orbital, mental and buccal branches of the fifth. The lips are especially sensitive near the rima oris.
Development of the lips and cheeks
During the second month in the human embryo, ledges of epithelium grow into the substance of the mandibular and the fused fronto-nasal and maxillary processes. These ledges develop into grooves which separate the upper and the lower lips from the upper and the lower jaws, the grooves forming the oral vestibule.
The philtrum and labial tubercle are said to correspond to the lower part of the fronto-nasal process. A failure of union between the globular and the maxillary processes presents an arrest of development resulting in the malformation known as "hare-lip".
In the late fetus and newborn, the red portion of the lips consists of an external smooth pars glabra, and an inner zone, pars villosa, which is covered with numerous villus-like projections. The largest of these reach a length of 1 mm. They also extend backward in an irregular band along the mucosa of the cheek. They disappear during the first few weeks of post-natal life.
In the infant, the corpus adiposum is especially well developed. On account of its supposed aid as a support for the buccinator in sucking, it has been called the "sucking pad."
The sebaceous glands of the mucosa are said not to appear until about the age of puberty.
Variations of the lips and cheeks
As is well known, the lips and cheeks are exceedingly variable in shape, size and structure in different individuals. There are also characteristic differences according to race and sex in the form and structure of the lips, rima oris, beard, etc. The "hare-lip" malformation was mentioned above.
Typical lips are found only in mammals, and are probably organs phylogenetically developed in connection with the process of suckling.
From Morris's treatise on anatomy.