222 COMPARATIVE ANATOMY. 



by powerful muscles, so that its secretion can be poured with 

 great force into the duct (Fig. 182, Gc), and thence into the 

 poison-fang (t). 



The sublingual gland of a Mexican Lizard, Heloderma, has a somewhat 

 similar poisonous nature. Its secretion passes out through four ducts, which 

 perforate the bones of the lower jaw in front of the grooved teeth. 



In marine Chelonians and Crocodiles there are no large glands united 

 into groups connected with the mouth. 



Birds. In Birds, and more especially in climbing Birds 

 (Scansores), well-developed lingual glands are present, opening 

 on the floor of the mouth. There is no doubt that they are 

 homologous with those of Lizards, and they probably correspond 

 with the posterior upper labial gland which opens into the angle of 

 the mouth, that is, to the poison-gland of Snakes. The palatine 

 glands of Birds have also their homologues in Reptiles. 



Mammals. Three glands may be distinguished in connection 

 with the mouth in Mammals, which are called, according to their 

 position: (1) parotid, (2) submaxillary, and (3) sublingual 

 (Fig. 172, Gls). Each opens by means of a well-defined duct (ducts 

 of Steno, Wharton, and Bartholini respectively) into the 

 mouth. The former corresponds to the gland opening into the 

 angle of the mouth in Birds, and consequently also to the poison- 

 gland of Snakes. As the last-mentioned is to be looked upon 

 as a differentiation of labial glands, the same origin must be 

 supposed for the parotid, and this is confirmed by a study of its 

 development. 



The fact that both the other salivary glands are homologous 

 with the sublinguals of lower Vertebrates needs no special proof, 

 and the numerous mucous glands lying at the sides of the tongue 

 and opening into the mouth come under the same category. 



Concerning the tonsils of Mammals, which lie at the junction of the 

 mouth and pharynx, compare pp. 239 and 293. 



Tongue. 



In Fishes and Dipnoi the tongue is, as a rule, rudimentary, 

 usually consisting simply of a covering of mucous membrane ex- 

 tending over the basal part of the hyoid, which in all the higher 

 Vertebrates serves as the main point of origin for the tongue- 

 muscles. The tongue is not capable of movement apart from the 

 visceral skeleton in Fishes, and, except in Cyclostomes (where it has 

 to do with the suctorial apparatus), is wanting in a proper muscula- 

 ture ; it is covered with papillae, and serves only as a tactile organ, 

 or, when covered with teeth, as a prehensile organ also. 



In the Perennibranchiata it remains in a similar condi- 

 tion to that seen in Fishes, but in all other Amphibia except 

 the Aglossa (Pipa and Dactylethra) it reaches a higher stage 



