106 THE VITAL FUNCTIONS. 



the Ditgong, or Indian Walrus, have teeth very differently 

 formed. The tusks of animals must necessarily, as respects 

 their shape, be classed among the conical teeth. 



273 



The sharp-edged teeth perform the office of cutting and 

 dividing the yielding textures presented to them; they act 

 individually as wedges or chisels, but when co-operating 

 with similar teeth in the opposite jaw, they have the power 

 of cutting like shears or scissors. The flat teeth, of which 

 the surfaces are generally rough, are used in conjunction 

 with those meeting them in the opposite jaw for grinding 

 down the food by a lateral motion, in a manner- analogous 

 to the operation of mill-stones in a mill. The tuberculated 

 teeth, of which the surfaces present a number of rounded 

 eminences, corresponding to depressions in the teeth op- 

 posed to them in the other jaw, act more by their direct 

 pressure in breaking down hard substances, and pounding 

 them, as they would be in a mortar. 



The position of the teeth in the jaws has been another 

 ground of distinction. In those Mammalia which exhibit 

 the most complete set of teeth, the foremost in the row have 

 the sharp-edged or chisel shape, constituting the blades of a 

 cutting instrument; and they are accordingly denominated 

 incisors. The incisors of the upper jaw are always im- 

 planted in a bone, intermediate between the two upper jaw 

 bones, and called the intermaxillary bones.* The conical 



* Those teeth of the lower jaw which correspond with the incisors of the 

 upper jaw, are also considered as incisors. In Man, and in the species of 

 quudrumana that most nearly resemble him, the sutures which divide the in- 

 termaxillary from the maxillary bones are obliterated before birth, and leave 

 in the adult no trace of their former existence. 



