mouth

mouth The poetic (and biblical) view of the mouth and lips is almost entirely romantic and idealized — ‘… the lips of a strange woman drop as a honeycomb, and her mouth is smoother than oil …’ (Proverbs), although an occasional writer illustrates the downside — ‘I've a head like a concertina, I've a tongue like a button stick, I've a mouth like an old potato, and I'm more than a little sick …’ (Kipling).

It is a rather unromantic fact, however, that the mouth is functionally the first part of the gastrointestinal tract — one end of the nutritional tube which starts there and ends at the anus. Because of its position it has acquired many other complementary functions — as a part of the respiratory system, for instance and as a most important part of the speech mechanism — but these are secondary to its main function. In the human embryo the first sign of the potential mouth (or oral cavity) occurs in the fourth week of development and is a small depression in what will become the skin of the face. The depression deepens and quite rapidly meets up with the developing upper part of the gastrointestinal tube. The separating tissues disappear, and the embryo mouth is left in continuity with the rest of the gastrointestinal tract. The oral mucous membrane includes many specialized features; the salivary glands, large and small, are derived from it, and it contains numerous sensory endings of various types. These include those in the taste-buds, the structures which are responsible (together with those in the nose mediating the sense of smell) for the recognition of flavour. Other types of nerve endings in the oral mucosa include those concerned with the sense of touch, recognition of temperature changes, and so on. In appropriate situations these provide signals to other parts of the body, stimulating the secretion of saliva, inducing gastric activity, initiating sexual awareness, and carrying out many other functions.

The bony structures within which the soft tissues of the mouth are contained are essentially the jaws (the mandible and the maxilla, including the palate) which, together with the precursors of the teeth, are formed as a later part of the developmental process described above. The system of embryonic structures involved in the formation of the mouth, lips, and jaws is complex, and the possibility of developmental errors occurring during this process is well known. In this highly visible and emotionally significant area of the body, failure of the normal processes, with resulting cleft formation, may be a highly traumatic matter for the individual involved. The integrity of the oral cavity and its relative proportions to the nearby structures, such as the nasal space and the sinuses, also largely determine the nature of speech, as the oral cavity is one of the series of resonators distributed about the base of the skull, which are greatly involved in modifying the primary speech (and song) sounds produced in the larynx.

In almost all of the functions of the mouth, the tongue and the teeth are closely involved. Perhaps less evident is the role of the saliva in this respect. As a lubricant with autonomic nervous system control of its flow (who has not had a dry mouth when subjected to almost any form of stress?), an adequate salivary flow is an absolutely vital factor for the success of most of the functions performed within the oral cavity.

The superficial margin of the mouth is marked by the lips — essentially the functional sphincter structure which seals off the mouth from the external environment, but with the ability to perform complex and sophisticated movements which quite transcend this simple function. The ring of individually controlled muscles in the substance of the lips, together with others in the facial structure, some of which are concerned predominantly with opening and closing the jaws, make up a highly complex system of control for the mouth, the lips, and the face in general. These include the group of the ‘muscles of expression’, which work with an integrated nerve supply to provide this vitally important mechanism of communication and of expression of emotion.

The mouth is, quite clearly, a primary erogenous zone. In itself it is not a particularly attractive structure, but the lips are a different matter. The smile is on the lips of the Mona Lisa — at least in the popular view, although quite clearly the facial expression as a whole is involved in such aesthetic assessments. ‘Thin lipped’, ‘thick lipped’ and similar characterizations depend on the description of only one feature of a face, but evidently a crucial one.

When things go wrong in the mouth the emotive effect may be disproportionally high. The mouth and lips are of great importance aesthetically, sexually, and functionally. Perhaps because of this, the area is also the site of many well-recognized psychogenic disorders involving unexplained pain and unusual sensations. Because of the duplex origin of the oral mucous membrane, diseases both of the skin and of the gastrointestinal tract, as well as those of more localized origin, may manifest in the mouth. Many of these cause pain, and the impact on the individual may be very significant — speech, eating, and the other functions in which the mouth is involved may all be affected, and the overall effect may be disproportionately great. Even minor conditions affecting the lips may be particularly troublesome — the simple cold sore causes distress, not only because of the irritation, but also because of its very visible site.

William Tyldesley

See also autonomic nervous system; cleft lip and palate; face; saliva; taste and smell; teeth; tongue.