406 ORGANS OF DIGESTION. 



cisors are shed twice in several rodentia, and most of the 

 teeth are renewed once in the other orders of mammalia. 

 Teeth are found in the lower jaw of the whale only in the 

 embryo state, and they are confined to this part in the adult 

 physeter. They are very deciduous in many of the cetacea, 

 edentata and cheiroptera, only two are developed in the front 

 of the lower jaw of the uranodon, the right upper incisor of 

 the monodon remains undeveloped in its alveolus, the upper 

 incisors are wanting in most ruminantia, and all the incisors 

 are deficient in the edentata, excepting the six-banded arma- 

 dillo ; but in the extinct anoplotherium alone among mam- 

 malia, the three kinds of teeth formed an uninterrupted 

 series around the jaws as in man. In most mammalia which 

 feed on animal matter, the crowns of the teeth are covered 

 entirely with enamel as they are likewise in the quadrumana 

 and man ; in the phytophagous quadrupeds, the enamel is 

 generally disposed in tortuous or waved vertical laminee, 

 which extend transversely or longitudinally according as the 

 motions of the jaws are longitudinal or transverse, as in 

 most rodentia, ruminantia and pachyderma, and in many 

 of the latter animals, as the elephant and capybara, several 

 of the molar teeth are aggregated together by a solid exter- 

 nal crusta petrosa to constitute a single compound masticat- 

 ing organ. 



From the increased strength of the jaws, and the greater 

 density and opposable character of the teeth, the temporal, 

 the masseter, and other muscles of mastication are stronger 

 than in the oviparous vertebrata, and from the lengthened 

 form of the muzzle, the muscles of the lips are more sepa- 

 rate and extended than in man, and produce more extensive 

 motions of the lips. The long moveable fleshy sensitive lips, 

 indeed, of most herbivorous quadrupeds serve them as hands 

 to collect and crop their vegetable food. The long flexible 

 tongue of the giraffe, the thumb-like process of the upper 

 lip of the rhinoceros, and the elongated nose or proboscis 

 of the elephant, are employed for the same purpose ; and it 

 is chiefly by their long vermiform lubricated tongue, that 

 the ant-eaters secure their insect prey. The retraction of 

 the highly moveable lips of carnivora serves to expose their 

 sharp teeth for free action, and so to intimidate and weaken 

 their prey. The numerous muciparous glands disposed 



