Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/956

Rh 896 ANATOMY [ORGANS OF SENSE move the entire tongue within the cavity of the mouth, protrude it between the lips, and again retract it, but can modify its form j thus the dorsum can be flattened, made convex or concave, the margins can be raised or depressed, and the tip elevated or depressed. I The mucous membrane of the tongue forms a part of the general mucous lining of the mouth; it covers the dorsum, tip, sides, and under surface; is reflected from the under surface to the floor of the mouth, where it forms ihefrcenum or bridle of the tongue, and is reflected also from the base to the epiglottis as the frcena of the epiglottis, as well as over the tonsils and anterior palatine pillars. This mem brane has its free surface elevated into multitudes of fine processes, called the papillae of the tongue, some of which are simple, others compound. The simple papillae are situated on the back part of the dorsum and the under sur face of the mucous membrane, as well as scattered between the compound papillae; they are simple conical elevations of the membrane. The compound papillae are arranged in three groups, named filiform, fungiforrn, and circumvallate papillae. The filiform papillce, elongated and thread-like, are the smallest and most numerous, and cover the dorsum in front of the circumvallate papillae. The fungiform or club-shaped are scattered over the anterior and middle parts of the dorsum, and at the tip and sides. The circumvallate papillce, seven to twelve in number, form a V-shaped figure on the dorsum towards its base ; a depres sion in the mucous membrane, called foramen caecum, marks the apex of the V. These are the largest papillse ; each is sunk in a vallum or trench-like depression of the mucous membrane, which isolates it from the surrounding surface. The compound character of these papilla} is due to each having projecting from it numerous small secondary papillae. The epithelial covering of the filiform papillae is characterised by the peculiar modification which the tessel lated epithelium of the mouth has undergone^ the cells have become cornified and elongated into dense, imbricated brush-like processes. In the carnivora the epithelium is so hardened as to form sharp spines, with the points turned backwards, which give to the tongues of these animals a rough prickly character. In the fungiform and circumval late papillae the inequalities between the secondary papillae, which project from them, are filled up by the tessellated epithelium, so that the surface of the compound papillae has a smooth appearance. Both the simple and compound Fio. 85. Section through a gustatory lamella of the rabbit s tongue, O. gusta tory bulbs situated in Kp, the epithelial layer of the mucous membrane ; V, capil lary blood-vessels in the sub-epithelial conuective tissue, (From a preparation by A. B. Stirling.) papillae are highly vascular; the lingual artery not only supplies the muscular substance of the tongue, but gives off fine branches to the mucous membrane. These branches end in capillaries, which form simple loops in the simple papillae, but in the compound papillae the capillaries are so multiplied that each secondary papilla has a capillary loop within it. The tongue is provided with several nerves. The hypo-glossal nerve supplies its muscular structure, but the inferior lingualis apparently receives a branch from the chorda tympani of the facial. The lingual branch of the fifth is distributed to the mucous membrane of the anterior two-thirds of the tongue : it breaks up into minute branches, which enter the fungiform and filiform papillae, but their exact mode of termination has not been precisely ascertained, though end-bulbs and gustatory bodies are said to have been seen in connection with some of the terminal branches. The glossal branch of the glosso-pharyngeal is distributed to the mucous membrane of the root of the tongue and of the circumvallate papillae. In connection with its terminal branches peculiar flask-shaped organs, called gustatory lulls or bodies, have recently been described by Lovdn, Schwalbe, and Engelmann, in the sides of the circumvallate papillae. These have been found in large numbers in lamellated folds of the mucous membrane of the posterior part of the side of the rabbit s tongue, which folds may appropriately therefore be called gustatory lamellae. When sections are made through one of these folds, or through a circumval late papilla and the trench which surrounds it, numerous flask-shaped gustatory bulbs may be seen in the epithelium, which covers the side of the papilla and the opposite side of the trench. The bottom of each flask is next the sub-epithelial tissue, whilst its short neck opens on the surface by a mouth, the gustatory pore ; similar bodies, though in much smaller numbers, have also been seen in the fungiform papillae. Each gustatory body consists of two diffe- Fic _ gc _^ snperficial rent forms of cells, named covering cells covering ceils of a and gustatory cells. The covering cells gustatory cenf witil are elongated, nucleated spindles, which, p, .&quot; peripheral, and ? ., f. c, its central process. arranged in layers, form the envelope 01 each gustatory bulb, and reach from the bottom to the mouth of the flask; they enclose the gustatory cells. The gustatory cells are attenuated, homogeneous, and highly refractile cells, which possess an elliptical nucleated body with two processes, a central and peripheral. These cells occupy the axis of the gustatory bulb. The peri pheral process, broader than the central, sometimes ends in a short hair-like tip, which almost reaches the gus tatory pore ; the central process extends to the base of the flask, and often divides into small branches. This Nerve process is varicose, and not unlike the axial cylinder taste. of a nerve fibre. The branches of the glosso-pharyngeal nerve, which are distributed to the back of the tongue, enter the circumvallate papilla?, and form a minute plexus, with groups of nerve cells interspersed in it, from which bundles both of medullated and non-meduUated fibres pass to the basis of the gustatoiy bulbs ; and it is believed that the finest uon-medullated fibres are continuous with the peripheral processes of the gustatory ceils, which are there fore regarded as the peripheral end-organs of the nerve of taste, and by the excitation of these bodies gustative or taste sensations are produced. As the glosso-pharyngeal is the nerve distributed to the circumvallate papillae, where these gustatory bulbs are especially found, it is therefore the special nerve of taste ; but as these bulbs have also been sparingly seen in the other papillae, where the lingual nerve is distributed, that nerve probably acts in a minor, ^ degree as a nerve of taste, though its special function is undoubtedly that of a nerve of touch. The gustatory -^ bulbs are not penetrated by blood-vessels, but, as Fig. 85 shows, the vascular sub-epithelial tissue is prolonged upwards along the sides of the bulbs almost as far as the plane of the gustatory pore. Key, Bealc, and other observers have described special modifications of the epithelium in connection with the terminations of the gusta tory nerves in the frog. The mucous membrane of the