TONGUE 327 



There is no doubt that the lingual glands are to a great extent 

 homologous with those of Lizards, but it is not known whether 

 the gland at the angle of the mouth corresponds with the posterior 

 upper labial gland of Reptiles that is, to the poison-gland of 

 Snakes. The median palatine glands of Birds are not homologous 

 with those of Reptiles, and labial glands are wanting. The lingual 

 glands are supplied by the glossopharyngeal, the others by the 

 trigeminal. 



Mammals. Three larger sets of salivary glands which have 

 become secondarily separated from one another may be distin- 

 guished in connection with the mouth in Mammals : these are 

 called, according to their position, (1) parotid, (2) submaxillary, 

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

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

 several independent ducts. A special retrolingual portion usually 

 becomes differentiated from the sublingual gland and commu- 

 nicates with the submaxillary duct. They all have a tubular or 

 tubulo-alveolar structure. 1 



The parotid is usually situated at the base of the external 

 ear. The submaxillary is a compound mucous and serous gland, 

 consisting of 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 sublingual gland extends between the 

 tongue arid the alveoli of the teeth, and is rarely absent (e.g. 

 Mouse, Mole, Shrew). 



With the exception of the parotid, the homology of which is 

 not clear, all these glands, together with certain smaller and less 

 important ones (buccal, lingual, palatine, and labial glands), are com- 

 parable to the oral glands of lower Vertebrates. 2 



Tongue. 3 



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

 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 or boring apparatus, the 



1 In some Mammals (e.g. Lepus) there is also an infraorbitcd or zygomatic 

 salivary gland, covered by the zygoma and extending into the orbit, its duct 

 opening anteriorly to that of the parotid. Salivary glands are wanting in the 

 Cetacea. 



2 The mucous glands are phylogenetically older than the serous glands, and 

 their essential function is merely to moisten the food. The serous glands have to 

 a greater or less extent become differentiated into an apparatus for producing a 

 secretion containing enzymes and acting chemically on the food. 



3 For the papillae of the tongue, cf. p. 254. 



