enamel, (b) rubbing upon the ivory, (c;) and the enamel, (b',) upon the crusta petrosa, (a,) of the two teeth opposite to it. Hence the act of mastication formed and perpetually maintained a series of wedges, locking into each other like the alternate ridges on the rollers of a crushing-mill; and the mouth of the Megatherium became an engine of prodigious power, in which thirty-two such wedges formed the grinding surfaces of sixteen molar teeth; each from seven to nine inches long, and having the greater part of this length fixed firmly in a socket of great depth.
As the surfaces of these teeth must have worn away with much rapidity, a provision, unusual in molar teeth, and similar to that in the incisor teeth of the Beaver and other Rodentia,[1] supplied the loss that was continually going on at the crown, by the constant addition of new matter at the root, which for this purpose remained hollow, and filled with pulp during the whole life of the animals.[2]
It is scarcely possible to find any apparatus in the mechanism of dentition, which constitutes a more powerful engine for masticating roots, than was formed by these teeth of the Megatherium; accompanied also by a property, which is the perfection of all machinery, namely, that of maintaining itself perpetually in perfect order, by the act of performing its work.
- ↑ The incisors of the Beaver, and other Rodentia, and tusks of the Hog and Hippopotamus, which require only an external cutting edge, and not a grinding surface, are constructed on the same principle as the cutting edge of a chisel or an adze; viz. a plate of hard enamel is applied to the outer surface only, of the ivory of these teeth, in the same manner as the outer cutting edge of the chisel and adze is faced with a plate of steel, welded against an inner plate of softer iron. A tooth thus constructed maintains its cutting edge of enamel continually sharp, by the act of working against the similarly constructed extremity of the tooth opposed to it.
- ↑ Pl. 5, Fig. 11, represents the section of the cavity containing this pulp.
