ALIMENTATION 93 



developed among ungulates unless used for defense or for digging 

 as in swine. The premolars and molars of the ox represent 

 a typical condition in which these teeth are adapted solely for 

 grinding. In such animals the jaw is articulated so as to permit 

 considerable side play, a motion that is familiar to anyone who 

 has watched a cow chewing its cud. 



Human teeth of the permanent dentition consist of four 

 incisors, two canines, four premolars and six molars in each jaw. 

 The incisors are spade-shaped with a cutting edge and occupy the 

 front of the jaws. The canines are conical and bluntly pointed. 

 The premolars and molars have flattened but not smooth biting 

 surfaces that are adapted for grinding. Human teeth are 

 adapted for an omnivorous diet. 



Tongue. — The tongue of fishes is simply a fold in the floor 

 of the mouth incapable of movement since it has no intrinsic 

 muscles. In some fishes it bears teeth. Above fishes the tongue 

 becomes a muscular organ appearing in a wide variety of morpho- 

 logical forms. The frog's tongue is attached at the margin of the 

 lower jaw and its free end when at rest is folded back on the floor 

 of the mouth. It is used in capturing food. In snakes the tongue 

 is used as a tactile sense organ. The tongue of mammals, 

 whales excepted, is a highly muscular organ capable of a variety 

 of movements. Its dorsal surface may, as in cats, develop 

 cornified, filiform papillae producing a rasping surface. The 

 tongue also may contain mucous glands and at its posterior end, 

 gustatory organs. 



Oral Glands. — Glands are lacking in the oral cavity of most 

 aquatic vertebrates. In air-breathing vertebrates glands are 

 present in a variety of forms. Such glands produce a secretion 

 that is poured into the oral cavity by means of ducts leading 

 from the glands. The principal oral glands of the frogs are 

 the intermaxillary glands, located chiefly between the pre- 

 maxillary bones and the nasal capsule and opening in the fore 

 part of the roof of the mouth by about 25 ducts. The secretion 

 of this gland gives the tongue its adhesive properties. 



The lining of the oral cavity consists of a ciliated columnar 

 epithelium, containing goblet cells, which produce a mucous 

 secretion. 



Mammals, in addition to numerous mucous glands located in 

 the oral epithelium, are provided with three kinds of salivary 



