TONG UE.
COUrSe of the mucous membrane.—The mu cous membrane of the mouth is continuous with that of the pharynx and larynx. Com mencing with the gums anteriorly, it passes upon the exterior surfaces of the upper and lower maxillary bones, and from thence is re flected on the cheeks laterally and upon the inner surface of the lips anteriorly, forming a small fold in the median line, called the frcenum, to each ; it invests the free borders of the lips and becomes continuous with the skin at a well defined line of demarcation. When the jaws and teeth are closed, the cheeks and lips are naturally in apposition with them ; but if sepa rated by distending the cheeks, the mucous membrane we have been tracing will be seen to line an anterior or second buccal cavity form ing a kind of antechamber to the interior of the mouth. Proceeding from the gurns posteriorly the membrane descends upon the interior of the lower jaw to be reflected upwards to the under surface of the tongue, and forms for it anteriorly and in the median line a prominent fold, the frcenum ; this occasionally is prolonged forwards to the apex of the tongue, interfering with its movements in the act of sucking : a slight division of the frcenum under these circumstances is all that is required. From the under surface of the tongue the mucous membrane invests that organ and is continued from its base to the epiglottis, and after forming three folds, called glosso-epiglottic, is reflected over its free edge to be continuous with the laryngeal membrane. From the gums of the upper jaw posteriorly it invests the hard and soft palate and passes round the posterior free border of the latter, after enclosing the uvula, to cover its nasal surface. From the cheeks laterally it is to be traced over the anterior pillars of the fauces, the internal surface of the tonsils dipping into its mucous crypts, and lastly forming the folds of the posterior pillars is continuous with the mucous membrane of the pharynx. Throughout the cavity of the mouth it is invested with epithelial scales, and its submucous areolar tissue is remarkably in creased in thickness and density when forming the gums and palate.
Funetion.—The pharynx, mouth, and palate are most obviously associated in the process of deglutition, inswhich we may trace three suc cessive stages : in the first, the food after being reduced to a softened pulp by mastication and admixture with the saliva is conveyed to the back part of the mouth by the movements of the tongue against the hard palate; this is a purely voluntary act and can be arrested at the will of the individual : the food carried past the anterior arch of the fauces, the second act of deglutition immediately succeeds ; this in volves the consentient action of numerous mus cles and is a most complicated process. If the
movements of the velum palati and the poste rior pillars of the fauces are examined during an effort to swallow, the former is perceived to become somewhat rnore arched towards the cavity of the mouth and to be rendered tense, but it appears to maintain nearly its naturally oblique direction. It has been supposed that the velum is raised during deglutition, in order to prevent the food from passing to the nose, but this opinion is now generally considered erroneous. 11/Iiiller says, " Most writers incor rectly state that during deglutition the food is prevented from entering the posterior nares by the soft palate being raised, a movement which, if performed, could not in any case completely cut off the pharynx front the posterior nares." With the stretching of the velum the posterior pillars or palato-pharyngei muscles will be seen to approach each other, particularly above, so as to reduce the isthmus faucium to a narrow triangular slit, broadest below. If the food is now pressed backwards by the tongue, it will be urged through this dilatable chink in a direction downwards and backwards,occasioned partly by the oblique resisting surface of the velum, and partly by the wider aperture left between the posterior pillars inferiorly, perhaps also by their greater disposition to yield in that direction to the pressure of the food as it passes between them ; meanwhile the pharynx (and the larynx with it) has been drawn up wards, and at the same time widened by the action of the stylo-pharyngei muscles, to receive the morsel, which in passing into it presses the epiglottis down upon the superior aperture of the larynx, and gliding over it is then immediately carried on to the cesophagus by the action of the constrictor muscles. The epiglottis in being shut down upon the opening of the larynx protects the respiratory tube, but it is not absolutely essential for that purpose ; ex periments have been performed on animals where the epiglottis has been removed, and it has been destroyed by disease in the human subject without any material difference evi denced in deglutition, the action of the laryngeal muscles closing the aperture of the larynx. This second act of deglutition may be performed at will though only the saliva is swallowed, but the effort soon becomes fatiguing. When the food, however, has reached beyond a cer tain limit in the mouth, no effort on our part can prevent deglutition from taking place. (For the influence of the nerves upon this function and that of taste see PAR VAGLIM, SPINAL ACCESSORY, GLOSSO-PHARYNGEAL.) The fur ther passage of the food through the cesophagus into the stomach (see (ESOPUAGUS) constitutes the third stage of deglutition and occurs invo luntarily.