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directed backwards, and corresponds to a depression on the surface of the tongue, the foramen cæcum, whilst its diverging limbs pass laterally and forwards towards the attachments of the glosso-palatine arch. The foramen cæcum is the remains of a tubular downgrowth formed early in embryonic life, in the floor of the primitive pharynx, from which the isthmus of the thyreoid gland is developed (see p. 44).
The anterior portion presents a velvety surface and is covered with innumerable papillæ; the taste-buds are situated in it, and it is horizontal in position. It is developed from the tissues of the floor of the pharynx behind the first visceral arch. The posterior portion, on the other hand, has a smooth glistening surface, contains numerous serous glands and small lymph follicles, and is more vertical in position. It is developed from the tissue covering the ventral ends of the second and third visceral arches (see p. 45).
The anterior or oral portion of the dorsum linguæ (Fig. 890) is convex, both from before backwards and from side to side in the resting condition of the organ (Fig. 893). It usually presents a slight median depression, sulcus medianus, in the form of an irregular crease, which ends posteriorly near the foramen cæcum. The mucous membrane of this portion of the dorsum is thickly covered with the prominent and numerous papillæ linguales which give this portion of the tongue its characteristic appearance.
On the pharyngeal part of the tongue there are also small papillary projections of the corium, but the epithelium fills up all the intervals between the papillæ, and, as it were, levels off the surface, so that none are visible to the eye as projections above the general level. Over the anterior part of the tongue, on the contrary, the projections of the corium are large and prominent, and the intervals between them, while they are covered, yet are not filled up, by the epithelium, so that the projections stand out distinctly and independently, and in places attain a height of nearly 2 mm. above the general surface.
The posterior or pharyngeal portion of the dorsum linguæ (Fig. 889), nearly vertical in direction, forms the greater portion of the anterior wall of the oral part of the pharynx (Fig. 890). Its surface is free from evident papillæ, but is thickly studded with rounded projections, each presenting, as a rule, a little pit, visible to the naked eye, at its centre; the great majority of these folliculi linguales (lingual follicles, Fig. 889), are similar to the lymph follicles found in the palatine tonsils; some few are said to be mucous glands; all are covered by a smooth mucous membrane, and they combine to give to this region a characteristic nodular appearance. To this collection of follicles the name tonsilla lingualis is applied.
The mucous membrane of this portion of the tongue is separated from the muscular substance by a submucous layer in which the lymph follicles and the mucous glands lie embedded (Fig. 893). At the sides it is continuous with the tunica mucosa covering the palatine tonsils and the side wall of the pharynx whilst posteriorly it is reflected on to the front of the epiglottis, forming in the middle line a prominent fold, the plica glosso-epiglottica (Fig. 889), at each side of which is a wide depression, the vallecula.
On each side is a pharyngo-epiglottic fold, which passes from the side of the epiglottis upwards along the wall of the pharynx, upon which it is soon lost.
Papilla of the Tongue (Fig. 891).—These are formed by variously shaped projections of the corium of the mucous membrane, covered by thick caps of epithelium. They are of three main varieties: 1, Conical and filiform (papillæ conica, p. filiformes); 2, Fungiform and lenticular (papille fungiformes et p. lenticulares and 3, Vallate and foliate (papilla vallate et p. foliata).
The conical and filiform papillæ (Fig. 891) are the smallest and most numerous forming as they do a dense crop of minute projections all over the anterior twothirds of the dorsum, and also upon the superior part of the margin and tip, of the tongue. Posteriorly they are arranged in divergent rows running laterally and forwards from the raphe, parallel to the limbs of the sulcus terminalis. More anteriorly, the rows become nearly transverse, and near the tip irregular. Each
papilla is composed of a conical projection of the corium, covered with microscopic papillæ like those of the skin, and covered by a thick long cap of stratified squamous epithelium.
In many of them the cap of epithelium is broken up into several long slender hair-like processes, giving rise to the variety known as filiform papilla. The cap of epithelium is being constantly shed and renewed, and an excessive or diminished rate of shedding or renewal, coupled with the presence of various fungi, gives rise to the several varieties of "tongue" found in different diseases.
The conical papillæ are longer and larger than the filiform, and have a wider base. They are situated on the dorsum among the filiform papillæ, and resemble them in their
The conical and filiform papillæ are probably of a prehensible or tactile nature, and are highly developed, and horny, in carnivora.
The fungiform and lenticular papillæ (Fig. 890) are larger and redder, but less numerous than the first variety, and they are found chiefly near the tip and margins of the tongue, comparatively few being present over the dorsum generally.
Each is in shape like a "puff-ball" fungus, consisting of an enlarged rounded head, attached by a somewhat narrower base. As in the case of the conical papillæ, the corium is studded over with microscopic papillæ, which are buried in the covering of squamous epithelium and do not appear on the surface. Most of the fungiform papillæ, if not all, appear to be furnished with taste-buds, and they are probably intimately connected with the sense of taste. The lenticular papillæ are placed on the margin of the tongue. They are flatter than the fungiform papillæ, and do not contain taste-buds.
The vallate papillæ (O.T. circumvallate) (Fig. 891), much the largest of all the papillæ of the tongue, are confined to the region immediately in front of the sulcus terminalis and foramen cæcum. Usually about nine to fourteen in number, they are arranged in the form of the letter V, with the apex posteriorly, just in front of and parallel to the sulcus terminalis. One or two of the papillæ are usually placed at the apex of the V, immediately anterior to the foramen cæcum. In appearance a vallate papilla resembles very closely the impression left by the barrel of a small pen pressed on soft wax (Fig. 891). Each is composed of a cylindrical
central part (1 to 2.5 mm. wide), slightly tapering towards its base, and flattened on its crown, which projects a little above the general surface of the tongue. This is surrounded by a deep, narrow, circular trench or fossa, the outer wall of which is known as the vallum. The vallum appears in the form of an encircling collar very slightly raised above the adjacent surface (Fig. 891).
As in the case of the other forms, the vallate papillæ are made up of a central mass of corium, studded with numerous microscopic papillæ on the crowns, but not on the sides, and covered over, as are the surfaces of the fossa and vallum, by stratified squamous epithelium. Into the fossæ open the ducts of some small
serous glands (Fig. 891 A).
On the sides of the vallate papillæ, as well as upon the opposed surface of
the vallum, are found, in considerable numbers, the structures known as taste-buds, the special end-organs of the nerves of taste.
Just anterior to the glossopalatine arch, on the margin, are usually seen about five or six distinct vertical folds, forming the folia linguæ, which are studded with taste-buds. They correspond to the papillæ foliata on the side of the tongue in certain animals Layer of muscle cut (rabbit, hare, etc.), in which they form an important part of the organ of taste.
Duct of the submax-
FIG. 892.-OPEN MOUTH WITH TONGUE RAISED, AND THE
The sublingual gland of the left side has been laid bare by the
gland of the right side a thin layer of muscle, in addition to the mucous membrane, has been removed. A branch of the lingual nerve is seen running on the medial aspect of the gland. The vena profunda linguæ is faintly indicated on this side also.
The apex and the margin of the tongue in front of the attachment of the glosso-palatine arch are free, and lie in contact with the teeth when the tongue is at rest.
On the superior half or more of the margin and apex, papillæ are present as on the dorsum; but on the inferior part they are absent, and the surface is covered by smooth mucous membrane.
The inferior surface of the tongue, which is exposed by turning the apex of the organ upwards, is limited in extent (Fig. 892) and is free from visible papillæ, smooth mucous membrane. Runthe surface being covered by & ning along its middle, except near the tip, is a depression, from which a fold of mucous membrane, the frenulum linguæ, passes down to the floor of the mouth, and on towards the posterior aspect of the mandible. side of the frenulum, and a short distance from it, the large profunda linguæ vein is distinctly seen through the mucous membrane. Further out still are situated two indistinct, fringed folds of mucous membrane, the plicæ fimbriata, which converge somewhat as they are followed forward towards the tip, near which they are lost.
From the inferior surface of the tongue the mucous membrane passes across the floor of the mouth to the medial surface of the gum, with the mucous covering of which it becomes continuous.
The plica fimbriata correspond pretty closely to the course of the deep lingual arteries as they
run towards the tip; the arteries, however, are deeply placed in the substance of the tongue, at a distance of 3 to 6 mm. from the inferior surface. The plicæ, which are more distinct at birth and in the fœtus, are said to correspond to the under tongue found in the lemurs.
The root of the tongue is the portion of the inferior aspect which is connected by muscles and mucous membrane to the mandible and hyoid bone. It is of very considerable extent, and is, with the base, the most fixed part of the organ. It is also the situation at which the vessels, nerves, and the extrinsic muscles enter.
Structure of the Tongue.-The tongue is composed chiefly of striped muscular tissue, with a considerable admixture of fine fat. A median septum of connective tissue occupies the central part of the organ. In addition, there are vessels, nerves, glands, and lymph tissue, the whole being covered over by mucous membrane, except at the root (Fig. 893).
The muscular tissue is derived partly from the terminations of the extrinsic musclesnamely, the hyoglossus, styloglossus, genioglossus, glossopalatinus, and chondroglossus; and also largely from the intrinsic muscles-namely, the longitudinalis superior, the longitudinales inferiores, the transversus linguæ, and the verticalis linguæ. These are so arranged that they form a cortical portion, made up chiefly of longitudinal fibres-derived, above, from the longitudinalis superior and the hyoglossus, at the sides, from
FIG. 893.-A, TRANSVERSE, AND B, LONGITUDINAL VERTICAL SECTION THROUGH THE TONGUE (Krause); C, A LYMPH FOLLICLE FROM POSTERIOR PART OF THE TONGUE. (Macalister, slightly modified.)
This cortex surrounds a
the styloglossus, and, below, from the longitudinales inferiores. central or medullary portion, divided into two halves by the median septum, and formed in great part by the transverse and vertical fibres, and also by the fibres of the genioglossi ascending to the dorsum. The muscular fibres derived from these various sources end by being inserted into the deep surface of the mucous membrane.
The detailed description of the extrinsic and intrinsic muscles will be found on
The septum is a median fibrous partition found in the medullary portion only, and easily exposed by separating the two genioglossi on the inferior surface of the tongue. Anteriorly it usually extends to the apex; whilst posteriorly it grows gradually narrower, and expanding transversely at the same time, it passes into a broad sheet (the hyoglossal membrane) which is united to the upper border of the hyoid bone, and gives attachment to the posterior fibres of the genioglossus. From the sides of the septum the fibres of the transverse muscle of the tongue arise.
The mucous membrane on the anterior two-thirds of the dorsum, and on the free margins, is firm and closely adherent to the underlying muscular substance, the fibres of which are inserted into it. On the posterior third of the dorsum, and on the inferior surface, it is neither so firm nor so closely united to the muscular substance, from which it is separated in both of these situations by a layer of submucous tissue.
The mucous membrane of the tongue, like that of the rest of the mouth, is covered by stratified squamous epithelium.
Glandulæ Linguales. Numerous small racemose glands are found scattered beneath the mucous membrane of the posterior third of the tongue; and a small collection of similar glands is present at the margin, opposite the vallate papillæ. Small serous glands are also found embedded in the dorsum near the vallate papillæ, into the fosse of which their ducts open (Fig. 891).
The chief collections of glandular tissue in the tongue, however, are found embedded in the muscle of the under surface, a little way posterior to the apex, on each side of the middle line (Fig. 892). They are known as the glandulæ linguales anteriores of Blandin or Nuhn.
These glands are displayed after the removal, from the under surface of the tongue, of the mucous membrane and a layer of muscle fibres about 2 mm. thick which is composed of fibres of the styloglossus and the longitudinalis inferior muscles a little distance behind the apex. The anterior lingual glands are oval in shape, often partly broken up by muscular fibres, and they measure from to in. (12 to 19 mm.) in length. They are mixed serous and mucous glands, and they open by three or four very small ducts on the inferior surface of the tongue.
FIG. 894.-LYMPH VESSELS OF THE TONGUE (after Poirier and Cuneo, modified).
Vessels. The chief artery is the lingual. This vessel passes forwards, on each side, medial to the hyoglossus muscle, and then is continued on to the apex-between the genioglossus on the medial side and the longitudinalis inferior laterally-under the name of the a. profunda lingua Anteriorly it is covered by the fibres of the longitudinalis inferior, and lies to in. from the surface. Near the apex the arteries of opposite sides are connected by a branch which pierces the septum; but otherwise, with the exception of capillary anastomosis, they do not com municate. The rami dorsales linguæ of the lingual artery are distributed to the pharyngeal part of the tongue, whilst some twigs of the ramus tonsillaris of the external maxillary artery are also distributed in the same region.
The veins are: The v. profunda linguæ, the chief vein, which lies beneath the mucous membrane at the side of the frenulum, and runs backwards over the hyoglossus with the hypoglossal nerve; two venæ comites, which accompany the lingual artery; and two dorsalis linguæ veins from the back of the tongue. These either unite and form a common trunk, or open separately into the internal jugular vein.
The lymph-vessels of the tongue take their origin in an extensive lymph network in the submucous coat, and a smaller network connected with the first, in the muscular substance of the tongue. The network at the apex, including the tip, margins, and front of the dorsum, is drained by some two to four vessels on each side, which pass downwards by the margin of the genioglossus muscle and pass laterally to the inferior deep cervical lymph glands. These vessels may be connected with the submental lymph glands also.
From the margins and dorsum of the tongue, behind the former area, and extending back to the vallate papillæ, lymph-vessels pass to the submaxillary lymph glands, and also, on the hyo