The palatine arch is formed, as we have already seen, by the upper jaw-bones and the palatine bones. It is circumscribed in front and on the sides by the upper teeth. It is covered with a thick mucous membrane, very hard, and presenting transverse ridges. Behind, it is continued by a musculo-membranous partition, called the veil of the palate— the soft palate—covered anteriorly by the mucous buccal, posteriorly by the pituitary membrane. Its inferior border is free and floating, presenting on the median line an appendage called the uvula. Each of its lateral borders forms a continuation with the tongue and the pharynx by two folds, which are called the pillars of the soft palate, and between which on either side lie the tonsils. In a state of repose the soft palate closes the back part of the mouth; but when raised prevents the food and drink, and the voice also, from passing into the nasal fossæ.
The tongue is a fleshy body, symmetrical, longer than it is broad, flattened from above downwards, thicker toward the middle than at its extremities, larger behind than in front, and rounded on the edges. The posterior extremity of the tongue is called its root, and the anterior the paint or tip. Its upper surface or dock, and a part of its edges, are covered over with papilla, which are divided into conical, fungiform, and cup-shaped papillæ. Its lower surface is free for about one-third of its length anteriorly: at the point of attachment we observe a mucous fold called the frœnum lingua or bridle of the tongue. Its two posterior thirds receive the muscles which fasten it to the neighbouring parts. The base or root of the tongue is fixed to the hyoid bone, an osseous semicircle bifurcated at its extremities, placed between the tongue and the larynx, and bound to these two organs by muscles, which gives unity to their movements in rising and falling. The tongue is formed of muscles, some of which are proper to itself, and others attach it to the hyoid bone, to the lower jaw, and to the styloid process of the temporal bone. Ail these muscles interlace their fibres in an inextricable manner, especially towards the upper portion of the tongue. At the median line and in the centre they are fixed to a cartilaginous plate, a sort of indirect prolongation of the hyoid bone, which gives greater solidity to the whole. The buccal mucous membrane covers the tongue, and is remarkably dense on its dorsal face.
The complex interlacement of the muscular fibres of the tongue permits a great variety of motion. It can raise or lower itself, lengthen or shorten, shrink or expand, diminish the end to a point, bend itself upwards and downwards, hollow itself into a canal lengthwise or breadthwise, carry its point and its edges to the parts of the mouth into which mastication has dispersed the food; in short, it exhibits in its movements and changes of form great force and the most subde dexterity.
The tongue receives three nerves; the great hypoglossal, the lingual, and glossopharyngeal; the first gives motion, and the last two are the sensitive nerves of taste. Under the influence of the first it takes part in the functions of digestion and in the articulation of sounds, and endowed by the others with a special sensibility, it is the principal organ of taste.
The bottom of the buccal cavity communicates with the pharynx, a canal with elastic walls formed by muscles, and lined with mucous membrane. It extends from the back of the mouth to the oesophagus, and is the vestibule of this passage, a sort of funnel, the upper part of which shares in deglutition, and adds to the resonance of the voice. The anterior wall of the œsophagus is formed by the larynx, the superior orifice of which, surmounted by the epiglottis, opens into the pharyngeal cavity, so that it is only the half of a canal completed in front by the larynx.
The Pharynx is continued below by the oesophagus, a tube formed by two membranes, the external muscular and the internal mucous. It is extensible and very contractile; it descends between the spinal column and the trachea, which it overlaps a little to the left, and on reaching the thorax it follows the posterior mediastinum, and at last traverses the diaphragm and opens into the stomach.