&c. An axe, or bill-hook, is not made entirely of steel, but of one thin plate of steel, inserted between two plates of softer iron, and so enclosed that the steel projects beyond the iron, along the entire line of the cutting edge of the instrument. A double advantage results from this contrivance; first, the instrument is less liable to fracture than if it were entirely made of the more brittle material of steel; and secondly, the cutting edge is more easily kept sharp by grinding down a portion of exterior soft iron, than if the entire mass were of hard steel. By a similar contrivance, two cutting edges are produced on the crown of the molar teeth of the Megatherium. (See Pl. 6, W. X. Y. Z. and Pl. 5, Figs. 6—10.[1])
Pl. 6, W. X. represents the manner in which each lower tooth was opposed to the tooth above it, so that the hard enamel of the one should come in contact only with the softer materials of the other; viz. the edges of the plates of
- ↑ The outside of the tooth, like that of an axe, is made of a comparatively sott material, viz. the crusta petrosa, (a a,) in closing a plate of enamel, (b b,) which is the hardest substance, or steel of the tooth. This enamel passes twice across the grinding surface, (z,) and forms the cutting edges of two parallel wedges, Y. b. b.: a longitudinal section of these wedges is seen, Pl. 6. v. w. x. y. Within the enamel, (b b,) is a central mass of ivory, (c,) which, like the external crust, (a) is softer than the enamel. A tooth, thus constructed of materials of unequal density, would have its softer parts, (a c,) worn down more readily than the harder plates of enamel, (b b.)
We find a further nicety of mechanical contrivance, for producing and maintaining two transverse wedges upon the surface of each tooth, in the relative adjustment of the thickness, of the lateral and transverse portions of the plate of enamel, which is interposed between the external crust, (a,) and the central ivory, (c.) Had this enamel been of uniform thickness all round the central ivory, the tooth would have worn down equally to a horizontal surface. In the crown of the tooth, Pl. 6. Z. the plate of enamel is seen to be thin on the two sides of the tooth, whilst the transverse portions of the same plate, (b. b,) are comparatively thick and strong. Hence the weaker lateral portions of thin enamel wear away more rapidly, than the thicker and stronger transverse portions, (b b,) and do not prevent the excavation of the furrow across the surface of the Ivory, c.
