mucous membrane covering the second and third visceral arches fuses to form the furcula (see RESPIRA TORY SYSTEM). Just in front of this a rounded eminence appears at an early date in the ventral wall of the pharynx to form the tuberculum impar which is separated from the furcula by the depression known as the sinus arcuatus. This tuberculum impar gradually grows to form the central part of the tongue in front of the foramen caecum, while the anterior part of the organ is derived from two lateral swellings which appear in the floor of the mouth and surround the tuberculum impar antero-laterally. The posterior third, or pharyngeal part, is developed from the anterior part of the furcula in the middle line, that is to say from the third visceral arch. The sinus arcuatus becomes gradually shallower as these two parts of the tongue grow together and eventually is indicated by the sulcus terminalis; in the mid line, however, the isthmus of the thyroid grows down from it, forming the thyro glossal duct the remains of which are seen in the foramen cae cum. (see DUCTLESS GLANDS). It will be seen that the tongue is developed in connection with the first, second and third visceral arches, and it is therefore to be expected that the fifth, seventh and ninth nerves which supply those arches would help to supply it, but the vagus from the fourth arch reaches it in addition, while the fact that most of the muscular substance of the tongue is supplied by the hypoglossal nerve is explained on the theory that some of the cervical skeletal musculature has grown cephalad into the tongue and has carried its nerve with it.
tongue is present in fishes but it is an immovable swelling in the floor of the mouth and is prac tically devoid of muscles. In the hag (31yxine) among the Cyclo stomata, and pike (Esox) among the Teleostei, teeth are developed on the tongue. In the Amphibia the tailed forms (Urodela) usu ally have tongues like fishes, though in the genus Spelerpes the organ is very free and can be protruded for a great distance.
In the majority of the Anura the tongue is usually attached close to the front of the floor of the mouth so that it can be flapped forward with great rapidity. There are, however, two closely allied families of frogs (Xenopodidae and Pipidae) which form the order of Aglossa, because the tongue is suppressed.
In the reptiles the tongue is generally very movable, though this is not the case in the Crocodilia and many of the Chelonia. The forked tongues of snakes and many lizards and the highly specialized telescopic tongue of the chameleon are familiar objects.
In birds the tongue is usually covered with horny epithelium and is poorly supplied with muscles. When it is very protrusible, as in the woodpecker, the movement is due to the hyoid, with the base of the tongue attached, moving forward.
In the Mammalia the tongue is always movable by means of well-developed extrinsic and intrinsic muscles, while papillae and glands are numerous. The filiform papillae reach their maximum in the feline family of the Carnivora where they convert the tongue into a rasp by which bones can be licked clean of all flesh attached to them.
Foliate papillae are best seen in the rodents, and when they are well developed the circumvallate papillae are few, often only one on each side.
In the lemurs an under tongue or sub lingua is found, which is probably represented by the plicae fimbriatae under the human tongue, and by some morphologists is regarded as the homologue of the whole tongue of the lower vertebrates, the greater part of the mammalian tongue being then looked upon as a new formation. For further details and literature see R. Wiedersheim's Compara tive Anatomy of Vertebrates, translated by W. N. Parker (London, 1907) ; C. Gegenbaur, Vergleich. Anat. der Wirbelthiere (Leipzig, 1901) ; A. Oppel, Lehrb. vergleich. mikroskop. Anat. der Wirbelthiere, Teil 3 (Jena, 1900) ; Parker and Haswell, Text Book of Zoology (London, 1897). (F. G. P.)