The tongue is almost entirely a muscular organ, which has the hyoid bone as a point of attachment, both for several of its muscles and for the hyo-glossal ligament. It plays an important part in mastication, keeping the food between the teeth with the help of the lips and cheeks, and in swallowing, in which act it guides the food backwards, and, pushing down the epiglottis, to which three folds of mucous membrane run (glosso-epiglottidean folds), covers over the upper end of the trachea. It is curious to note that the soft yielding tongue is a potent factor in moulding the rigid jaw, which latter, after excision of the tongue, frequently falls in. While the tongue certainly plays an important part in speech, it is by no means essential for that purpose, patients frequently being able to speak excellently after excision of the tongue, and even swallowing is managed with practically no difficulty. The mucous membrane of the tongue presents on the dorsum anteriorly a large number of filiform and a less number of fungiform papillae, and posteriorly a row of circumvallate papillae arranged in a V-shape, with the apex directed backwards. The foramen ccecum is situated at the apex, while the limbs stretch out and forwards toward the anterior palatine arches. Behind the circum vallate papillae it is smooth but nodular, owing to the presence of the lingual tonsil beneath the surface. This posterior third of the tongue is almost vertical, and forms the anterior wall of the oral pharynx. When swollen, it may interfere with the action of the epiglottis, to which it is connected by a median fold of mucous membrane, the frenulum epiglottidis. The mucous membrane of this region is continuous laterally with that of the tonsils and pharynx. In many debilitated conditions the surface epithelium of the tongue accumulates and becomes sodden and stained, producing a furred tongue ; while the strawberry tongue of scarlet fever is due to the bright red colour of the sparse fungiform papillae scattered throughout the fur. While the surface normally is fairly smooth, save for the median raphe, it becomes much fissured in syphilis of the tongue, and is frequently associated with leucoplakia, in which the villi are generally destroyed and the epithelium becomes thick and white, resembling white oil-paint. The foramen caecum is a remnant of the thyro-glossal duct, from which cystic tumours, some of which are malignant, and dermoids have developed. The latter have occasionally been so large as to protrude the tongue from the mouth.
The under surface of the tongue is covered with smooth, comparatively lax mucous membrane, and presents a median fold of mucous membrane, the frenulum lingua, which extends from it to the floor of the mouth toward the lower jaw. This frenulum is sometimes very short, giving rise to the condition known as ' tongue-tie,' which, when pronounced, may cause the infant difficulty in sucking. If it be necessary to cut it. this should be done with care close to the jaw, as a free incision may wound the ranine vessels, and even permit the tongue to turn over into the pharynx. On either side of the frenulum the ranine vein may be seen, lying near the surface, while the arteries run somewhat parallel, but much more deeply. The tongue is richly supplied with blood, and is a frequent seat of navi. In addition to the lingual vessels (which send off a dorsal branch proximal to the usual place of ligature in the neck, which supplies the root of the tongue and the tonsil), it receives blood by anastomosis with small branches of the ascending pharyngeal and facial arteries. The tongue possesses a rich lymphatic supply, the channels from the posterior half following the ranine vein to discharge into the deep cervical glands beneath the sterno-mastoid and angle of the jaw, while those from the anterior portion (that most frequently affected by carcinoma) pass through the mylo-hyoid muscle into the submaxillary glands. A great increase in the lymphatics, occurring congenitally, is known as macroglossia. It is a rare condition, and the tongue has become so much enlarged in such cases as to protrude several inches from the mouth, and even cause dislocation of the jaw. The tongue is occasionally affected by parenchymatous glossitis, in which it becomes greatly swollen, possibly protruding between the teeth, and tending to cause suffocation by backward pressure and oedema of the aryteno-epiglottic folds.
The nerve-supply of the tongue is rather complex. The twelfth is the motor nerve, supplying all the muscles (except the palato-glossus, which is supplied by the pharyngeal plexus). This nerve leaves the skull by the anterior condylar foramen, at which point it has been damaged by injury to the back of the head, giving rise to atrophy of half of the tongue. The lingual (fifth) supplies the anterior two-thirds of the tongue with common sensation, and tactile sensation is more sensitive at the tip of the tongue than in any other part of the body. In painful affections of the anterior portion of the tongue pain is frequently referred to the auditory meatus and skin of the surrounding parts (supplied by the auriculotemporal branch of the fifth). The lingual may also supply taste to this portion, but it is more probably due to the chorda tympani (given off by the facial in the aqueduct of Fallopius), and in support of this view is the fact that the destruction of the chorda in the aqueduct has been followed by loss of taste in the tongue on that side, while stimulation caused sensations of taste. The chorda has also an effect on nutrition of the tongue (including the circumvallate papillae). The epiglottis and base of the tongue are partially enervated by the internal laryngeal (of tenth).
The muscles oj the tongue may be considered in three groups -intrinsic, extrinsic, and accessory. The intrinsic are composed oi a cortical layer of longitudinal fibres (with which the hyo- and stylo-glossi blend) and a central core of transverse and vertical fibres (with which fibres of the genio-glossi blend), which are divided into two lateral masses by the septum. The extrinsic consist of the hyo-, genio-hyo-, stylo-, and palato-glossi muscles, which largely compose the root of the tongue. The accessory consist of the muscles which act indirectly on the tongue through the hyoid bone-omo-, mylo-, genio-, stylo-, and sterno-hyoid, and digastric.