The tongue [latin: lingua; French: La langue] is a muscular organ covered with mucous membrane and located in the floor of the mouth. It is an important organ of mastication, deglutition, taste and speech. Upon its upper surface is a V-shaped groove (sulcus terminalis) indicating the division of the tongue into two parts. The larger anterior part, or body [corpus linguae] belongs to the floor of the mouth, while the smaller posterior part, or root [radix linguae], forms the anterior wall of the oral pharynx. The inferior surface (facies inferior) of the tongue is chiefly attached to the muscles of the floor of the mouth, from the hyoid bone to the mandible. Anteriorly and laterally, however, the inferior surface of the body is free and covered with mucosa. The superior surface of the body is called the dorsum. It is separated from the inferior surface by the lateral margins, which meet anteriorly at the tip [apex linguae].
The dorsum of the tongue usually presents a slight median groove [sulcus medianus linguae]. Its posterior end corresponds to a small pit of variable depth, the foramen caecum, which is placed at the apex of the V-shaped terminal sulcus. The dorsum of the body has a characteristic rough appearance due to numerous small projections, the lingual papillae.
1. Lingual papillae
Five or six varieties of papillae are distinguished, between which intermediate forms occur. The conical [papillae conices] and thread-like [papillae filiformes] are most numerous, and are arranged more or less distinctly in rows parallel with the terminal sulcus. They are best developed toward the midline of the dorsum in its posterior part. As shown in vertical section, each papilla consists of an axial core of vascular fibrous tissue (from the lamina propria) often beset with smaller secondary papillae. The stratified squamous epithelial covering often presents numerous thread-like prolongations from the apex of the papilla. The papillae vary from 1 to 3 mm. in length.
The fungiform ("toad-stool shaped") papillae are somewhat similar to the conical in structure, but larger and more prominent, with an expanded free portion and a slightly constricted stalk of attachment. They are relatively few in number and are scattered irregularly over the dorsum, being most numerous near the margins. They are easily distinguished in life by their larger size and reddish color. A smaller, flattened variety of the fungiform is sometimes called the lenticular ('lens-shaped') papillae. (This term, however, is applied by Toldt to certain small rounded-elevations with underlying lymphatic nodules in the mucosa of the root of the tongue.)
The vallate (circumvallate) papillae, usually seven to eleven in number, are conspicuous and arranged in a V-shaped line parallel with and slightly anterior to the sulcus terminalis. They are, as a rule, shaped like short cylinders, 1 to 2 mm. in width, and somewhat less in height. As is shown in section, each is surrounded by a trench or fossa, into the bottom of which open ducts of the serous glands of von Ebner. On the sides of the fossae are the taste-buds, as described in the section on Sense Organs.
The foliate papillae are represented by a few (five to eight) parallel transverse or vertical folds of mucosa, along the margins of the tongue just anterior to the glosso-palatine arch on each side. They are variable in size and sometimes rudimentary. In structure they somewhat resemble the vallate papillae (though of different form), their walls being studded with taste-buds.
The free inferior surface of the tongue is covered by a thin smooth mucosa. In the median line is a prominent fold, the frenulum, which connects the tongue with the mandible and the floor of the mouth. On each side of the inferior surface, an irregular, variable, fringed fold, the plica fimbriata, extends from near the apex backward approximately parallel with the lateral margin of the tongue. Between the frenulum and the plicae fimbriatae, the lingual (ranine) veins are visible on each side beneath the mucosa.
The root (or base) of the tongue [radix linguae] belongs to the pharynx, but is here included with the mouth for convenience of description. Its free surface is directed posteriorly, and represents the continuation of the dorsum linguae. Laterally it is continuous with the region of the palatine tonsils. Infe riorly it extends to the epiglottis, with which it is connected by a median and two lateral folds, between which are the depressions known as the valleculae. The mucosa over the root of the tongue is irregular and warty in appearance due to the projections of the underlying nodular masses of lymphoid tissue, the lingual follicles. A crypt or tubular pocket of surface epithelium usually dips down into each of these follicles, and shown in section. The follicles vary from 34 to 102 in number, the average being 66 (Ostman) j and are somewhat irregular in size and form. They are often arranged in more or less distinct longitudinal rows, with corresponding folds of the mucosa (Jurisch). The lingual follicles are collectively designated as the lingual tonsil [tonsilla linguae]. Between the lingual follicles and around the periphery of the lingual tonsil there are found smaller ordinary nodules (without crypts) and indefinite masses of lymphoid tissue. The sulcus terminalis forms a fairly sharp boundary between the lymphoid mucosa of the root and the papillated mucosa of the body of the tongue.
2. Glands of the tongue
The glands of the tongue are of three types - mucous, serous and mixed. The most numerous are those of the mucous type, which are typical for the mouth cavity in general and resemble those already described in the lips, cheeks and palate. They are spread over the entire surface of the root of the tongue, in the spaces between the lingual follioles, usually opening upon the surface but in many cases into the crypts. Anteriorly, they extend a short distance along the posterior portion of the lateral margin of the tongue, and also occupy small areas in and near the midline in front of the vallate papillae.
In the immediate region of the vallate papillae, and in the small lateral areas corresponding to the foliate papillae (i. e., in the regions of the taste-buds), the mucous glands are displaced by the serous glands (of von Ebner), which have a watery secretion. Finally, on the inferior surface of the tongue, on either side of the frenulum near the apex, are the anterior lingual glands (glands of Nuhn or Blandin). Each is about 15 mm. in length, and is composed of a group of racemose glands with three or four very small ducts opening on the surface of the tongue near the plica fimbriata. The anterior lingual glands are deeply placed and are covered not only by the mucosa, but also by some of the longitudinal muscle fibers (inferior longitudinal and styloglossus). This gland is of the mixed type, though chiefly mucous.
3. Muscles of the tongue
A layer of fibrous connective tissue, the lingual septum, separates the halves of the tongue, extending in the median plane from the apex to the root, where it is attached below to the hyoid bone. The muscles of the tongue are classified as extrinsic and intrinsic. The extrinsic muscles extend into the tongue from without. They are the hyoglossus, chondroglossus, genioglossus, styloglossus, and glossopalatinus (palatoglossus), all of which are described elsewhere.
4. The intrinsic muscles
The longitudinalis superior is a superficial longitudinal stratum extending from the base to the apex of the tongue, immediately beneath the mucosa of the dorsum, to which many of its fibers are attached. The longitudinalis inferior is composed of two muscle-bands extending from base to apex on the inferior surface of the tongue, and is situated between the hyoglossus and the genioglossus, some of its fibers near the apex mixing with the styloglossus, while dorsally some are attached to the hyoid bone. The transversus linguae consists of fibers which pass transversely, and is situated between the superior and inferior longitudinal muscles. The fibers arise from, or pass through, the septum linguae, and are attached to the mucosa of the dorsum and lateral margins of the tongue. The verticalis linguae is composed of fibers which pass from the mucosa of the dorsum to the mucosa of the inferior surface of the tongue, interlacing "with those of the other intrinsic and extrinsic muscles.
5. Vessels and nerves of the tongue
The lingual arteries furnish the principal blood-supply. The lingual veins carry the blood from the tongue to the internal jugular. The lymphatics form a network in the lamina propria, connected with a deeper network in the submucosa. The latter forms plexuses around the lingual follicles. The efferent lymph-vessels from the tongue empty chiefly into the superior deep cervical lymph-nodes. The nerves are motor and sensory. The hypoglossal nerve supplies the intrinsic and all the extrinsic muscles of the tongue except the glossopalatinus (palato-glossus), which is supplied from the pharyngeal plexus. The sensory nerves are: - the lingual nerve, a branch of the mandibular division of the fifth, which, after joining with the chorda tympani from the seventh, is distributed to the anterior two-thirds of the tongue and represents the nerve of touch; the lingual branches of the glossopharyngeal, which are distributed to the root of the tongue, including also the vallate and foliate papillae (nerve of taste) ; and the superior laryngeal branch of the vagus, which supplies a small area near the epiglottis.
6. Development of the tongue
The development of the tongue is quite complicated. In general, the body of the tongue is derived from the region corresponding to the ventral portion of the first arch, just behind the mandible. It does not develop from the tuberculum impar, however, which is a transitory structure (Hammar). The epithelium of the body of the tongue is probably of ectodermal origin. The root of the tongue develops from the corresponding lower portion of the second or hyoid arch, and its epithelium is endodermal in origin. The transverse groove between the two arches later becomes the sulcus terminalis. At the middle of this groove there is an ingrowth of the epithelium to form the anlage of the thyroid gland. The foramen caecum and the occasional ductus lingualis represent persistent portions of the thyroid duct. The third arch does not appear to enter into the formation of the tongue, but forms the epiglottis (Hammar).
The musculature of the tongue appears to develop from the mesenchyme in situ although its innervation from the hypoglossal would indicate a derivation from the occipital myotomes. A pair of premuscle masses appears in the 9 mm, embryo, the individual extrinsic muscles being distinguishable at 14 mm., and the intrinsic at 20 mm. (W. H. Lewis). The glands appear in the fourth fetal month as solid epithelial downgrowths which later acquire a lumen. The mucous glands appear first, the serous slightly later. Longitudinal folds in the mucosa of the radix appear in the third or fourth fetal month (Jurisch). The lymphoid tissue appears somewhat later as aggregations in the lamina propria, chiefly around the gland-ducts. From the beginning, the lymphoid structures are subject to marked individual variations. Characteristic, well-developed lingual follicles do not appear until sometime after birth, however (Jurisch). Of the lingual papillae, the fungiform appear at the end of the third fetal month, followed shortly by the filiform and vallate. The formation of the papillae is not completed at birth, however, since they later undergo changes in number, form, size and arrangement. The foliate papillae appear about the fifth fetal month. They are best developed in infants, under-going retrogressive changes in the adult (Stahr). The same is true of the plicae fimbriatae.
Of the manifold variations in the structure of the tongue, some have already been mentioned. Additional mucous glands sometimes occur along the margin of the tongue (completing Oppel's "glandular ring"). In "tongue-tied" individuals, the frenulum is abnormally short. A forked tongue (normal in some animals) is a rare congenital anomaly. Another rare variation is the so-called "hairy" tongue, due to hypertrophy of the filiform papillae. While the V-shaped arrangement of the vallate papillae is typical, the Y-form (two to four papillae in the median line forming the stem of the Y) is nearly as frequent. Indeed, in some of the colored races the latter type seems to predominate. The sulcus terminalis and foramen caecum are often indistinct and sometimes absent.
The tongue of fishes and lower amphibia contains neither glar.ds nor intrinsic musculature. Among higher vertebrates, the tongue varies exceedingly in form and structure, but always contains intrinsic musculature and mucous glands. The latter primitively form a ring around the margin and root of the tongue (Oppel). The serous glands occur only in mammals, and are associated closely with the papillae bearing taste-buds.
The plica fimbriata in man is homologous with the 'sublingua' of lower mammals. According to Gegenbaur, the 'sublingua' represents the entire primitive vertebrate tongue, but this view is opposed by Oppel. Among various mammals, the number of vallate papillae varies from one to thirty, but the V- or Y-arrangement is typical. The region of the foliate papillae ('marginal organ') is typical for mammals, and is much better developed in some (e. g., rabbit) than in man. The mucosa of the root of the tongue is always different from that of the body. The lingual papillae are especially developed in the tongue of carnivora.
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