Oral morphology and tongue habits.
Human beings ingest food, transform the food into the energy needed for existence, and communicate through speech, thus forming society. The oral cavity plays an essential role in these functions. When ingesting food, appropriate biting, mastication and swallowing must be conducted, and when speaking, words must be pronounced accurately. There is no doubt that the cooperative movements of the lips, teeth and tongue form the principal axis for each of these activities. The positions into which the teeth naturally erupt and align themselves are the points of balance between the opposing forces exerted by the tongue and by the lips. For instance, if the forward extension of the tongue is strong and the tension of the lips is weak, the teeth will protrude slightly forward; conversely, if the strength of the lips is strong and that of the tongue is weak, it is believed that the teeth will be inclined toward the direction of the tongue. Also, as the tongue is a mass of muscle, its shape at rest is often a manifestation of its functional characteristics. Likewise, the lips are composed of tissue surrounding muscle, and thus the shape of the lips can often reveal their function. Based on these observations, we conducted a survey of the relationships between the shape and form of the tongue and lips, versus the inclinations of the dental axes of the front teeth of the upper and lower jaw.