252 COMPARATIVE ANATOMY 



fibrous sheath, and is acted upon by powerful muscles, so that its 

 secretion can be poured with great force into the duct (Fig. 201), 

 and thence into the poison-fang (p. 243). 



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

 poisonous nature. The secretion passes out through four ducts, which 

 perforate the bones of the lower jaw in front of the grooved teeth (p. 245). 



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), a well-developed lingual gland is present opening on 

 the floor of the mouth, as well as a gland at the angle of the 

 latter. There is no doubt that the lingual glands are homologous 

 with those of Lizards, but it is not known whether the gland at the 

 angle of the mouth corresponds with the postei'ior upper labial 

 gland of Eeptiles — that is to the poison-gland of Snakes. The 

 palatine glands of Birds are not homologous with those of Reptiles 

 and labial glands are wanting. 



Mammals. — Three sets of salivary glands may be distin- 

 guished in connection with the mouth in Mammals, which are 

 called, according to their position, (1) pao'otid, (2) submaxillary, 

 and (3) suMingual. Each of the two former of these opens into 

 the mouth by a well-defined duct, that of the sublingual having 

 several independent ducts. A special retrolingital portion usually 

 becomes differentiated from the sublingual gland and commu- 

 nicates with the submaxillary duct. 



The parotid is usually situated at the base of the external 

 ear : its origin is not known. The submaxillary is a compound 

 gland, consisting of glandular elements which differ from one 

 another histologically : it lies beneath the mylohyoid muscle, close 

 to which the retrolingual gland is also situated ; the latter is 

 wanting in only a few Mammals {e.g.. Rabbit, Horse). The sub- 

 lingual gland extends between the tongue and the alveoli of the 

 teeth, and is rarely absent {e.g., Mouse, Mole). 



With the exception of the parotid, all these glands, together 

 with certain smaller and less important ones, are homologous with 

 the oral glands of lower Vertebrates. 



Salivary glands are wanting in the Cetacea. 



Tongue. 



Fishes. — The tongue is, as a rule, rudimentary in Fishes, and 

 is simply represented by a fold of mucous membrane covering the 

 basi-hyoid, which in all the higher Vertebrates serves as a point of 

 origin for many of the lingual muscles. Except in Cyclostomes, 

 where it has to do with the suctorial apparatus, the tongue of 



