The Lip Muscles

The orbicular is orris closes the lips and, when contracting, its lateral insertions into the modioli are fixed by the contraction of the quadratus labii superioris and inferioris, zygomaticus, triangular is, and buccinator muscles. The incisal muscles also fix it to the sulci of the alveolar ridge.
During such function there is considerable inward pressure on the labial surfaces of the teeth and the labial sulci are reduced in depth. The upper denture, due to its better stability, can usually withstand such pressure provided its labial flanges have been designed correctly. The lower denture , however, particularly in cases presenting poorly formed alveolar ridges, is likely to be raised from the ridge in front and pushed backwards. Experienced denture wearers counteract this pressure of the lip by a forward pressure of the tongue but assistance can often be given to the tongue by carrying the heels of the lower denture up the ascending ramie of the mandible on such cases. The distal extension of the lower denture are often referred to as posterior stops.

Function of Tongue

Excluding taste, these are twofold. It controls the food during mastication and swallowing. It controls and directs, with the aid of the lips, teeth and palate the vibrating airstream from the larynx to form the sounds of articulate speech.
During mastication the tongue performs as follows. As the piece of food is being incised the tip of the tongue controls and steadies it. In the case of food which does not require incising the tongue is protruded slightly and its center depressed to form a shallow concavity into which the food is placed.

The Tongue

Rest Position of the Tongue

The dorsum rests against the roof of the mouth and the tip of the tongue rests in contact with the lingual surfaces of the lower incisor teeth.
The lateral borders lie against the lingual borders of the posterior teeth and protrude slightly into the free-way space between the occlusal surfaces of the upper and lower teeth.
Posteriority the soft palate rests on the dorsum of the tongue during normal nasal breathing.
When the teeth are extracted the tongue spreads laterally and the lips and cheeks fall in to meet it so filling between them the space left by the teeth, sometimes termed the neutral zone or zone of minimal conflict.