TONGUE. 223 



and becomes larger in size by the development of definite 

 muscles in connection with it. Its mobility varies greatly in the 

 different groups of Amphibia in accordance with the manner 

 in which it is fixed to the floor of the mouth. It is usually 

 attached only by the anterior end or by a portion of its ventral 

 surface ; in other cases it is free all round, and in Spelerpes 

 (Fig. 183) is capable of being extended far out of the mouth by 

 means of a complicated mechanism. 



FIG. 183. HEAD OF Spe lerpes fuscus, WITH THE TONGUE EXTENDED. 



In most Reptiles and Birds the tongue is freely moveable, 

 but its form and relative size varies greatly in the different families 

 (see Fig. 184, A to D). It is least mobile in Chelonians and 

 Crocodiles : 1 in Chameleons, on the other hand, it is very long and 

 protrusible. 



The tongue of Birds which is usually poorly provided with 

 muscles, may be looked upon as having been derived from a 

 similar form to that of Lizards, and its shape as a rule corresponds 

 more or less to the form of the beak. It possesses a horny cover- 

 ing, usually provided with papillae and pointed recurved processes ; 

 as in many Reptiles, it may be split up at its distal end, being 

 either bifurcated (Colibris) or having a brush-like form. In Wood- 

 peckers (the extraordinarily developed epibranchials of which have 

 already been mentioned in the chapter on the skull), the tongue 

 may be thrown far out from the mouth by means of a complicated 

 system of muscles, and it thus serves as a prehensile organ; in 

 this Bird and in the Duck it is richly provided with Pacinian 

 corpuscles. The tongue is largest in predatory Birds (Rapaces) 

 and Parrots, but its size is here not due so much to the special 

 development of muscles as to the presence of fat, vessels, and 

 glands. 



The tongue reaches its most complete development in Mammals, 

 and, as elsewhere, undergoes the most various modifications as 

 regards size, mobility, and function, according to the method of 

 taking in food. It is as a rule flat, and rounded anteriorly, having 

 a band-like form, and being extensile. A fold, the so-called sub- 

 lingua (plica fimbriata and mediana), is present on its lower 

 surface. This represents a primitive organ which must be regarded 

 as the predecessor of the structure which we now speak of as the 

 tongue. 



In spite of the various functional modifications of the tongue 



1 The relative importance and degree of development of the tongue does not run 

 parallel with the systematic position of the animal. 



