116 COMPAEATIVE DENTAL ANATOMY 



armadillo. There are usually four limbs present, 

 hence they are called quadrupeds, — four-footed 

 animals. 



The heart is four-chambered and the blood 

 warm. Eespiration is performed by cellular 

 lungs. The osseous system is more rigid and 

 stronger than that of the lower Vertebrates, ow- 

 ing to the coalescence of bones which remain dis- 

 tinct in the latter, and to the greater solidity due 

 to a larger quantity of lime-salts. The bones of 

 the skull and jaws are more solidly united, and 

 the numbers of bones is reduced. The mandible 

 arises from distinct centers of ossification (which 

 represent distinct bones in the fishes and reptiles), 

 but it usually consists of a single bone in the adult 

 mammal. In most of the lower mammals the su- 

 perior maxillaries are more elongated than in 

 man, and the intermaxillary bones remain dis- 

 tinct through life. In man they are fused with 

 the maxillaries. 



The teeth of Mammals present great variety, 

 as the food employed is varied, and highly spe- 

 cialized teeth are developed to accommodate the 

 food habits. Some low forms are destitute of 

 teeth, but nearly all mammals are possessed of 

 true teeth. The offices of the teeth are also ex- 

 tended in this class, for while in the fishes and 

 reptiles the teeth are mainly employed for the 

 purpose of prehension only, in the mammals spe- 



