904 THE SHAPES OF TEETH [ch. 



in which the enamel-layer which covered the hills and lined the 

 valleys is seen in sectional contour. A horse's incisor is the simplest 

 case. On its worn surface we see an inner ring of enamel concentric 

 with the enamel of the outer edge or surface of the tooth; cement 

 fills up the inner ring, and dentine the space between. The tip of 

 the tooth has sunk down, or been tucked in, till it forms a cement- 

 filled lake on the top of the hill; the lake narrows in, and at last 

 vanishes as the horse grows old and the tooth wears down; in the 

 "aged" horse we see the "mark" no more. To recognise this lake 

 or pit in the simple contours of the young incisor is an easy matter ; 

 but in the abraded molar the enamel-layer which once covered all 

 its ups and downs forms a contour-line, or "curve of level," of great 

 complexity. This contour-line alters as the levels change, and varies 

 from one tooth to the next and from one year to another, so long as 

 wear and tear continue. The geographer reads the lie of the land, 

 with all its ups and downs, from a many-contoured map*, but the 

 worn tooth shews us only one level and one contour at a time ; we 

 must eke out its scanty evidence by older and younger teeth in other 

 phases or degrees of wear. The " pattern " of a horse's molar tooth is 

 indeed so closely akin to a map-maker's contours that some of the 

 terms he uses may be useful to us. He speaks, for instance, of ridge- 

 lines and course-lines, lignes defaite and lignes de thalweg; of a gap, 

 or lowland way between two hills, in contrast to a col or saddle at 

 the summit of a mountain-pass; or of a gorge, which is a narrow 

 steep-sided valley; or a scarp, which is a long steep-faced hillside. 

 We must take care all the while to see which side of our contour-line 

 is positive or negative — on which side the ground slopes up and on 

 which down. In our tooth we find that every enamel-contour has 

 dentine on the one side and more or less cement on the other; the 

 dentine belongs to the closed interior of the tooth itself, and on the 

 other side of the enamel-line are spaces open to the world. 



In a horse's molar we see the sinuous contours of two small lakes, 

 remains of the two valleys which lay between the three transverse 

 ridges of the compound tooth; and outside the enamel-edges of 



* Contour-lines or horizontals, as some geographers prefer to call them, were 

 invented by Buache, in 1752. These are discussed by Cayley, On contours and 

 slope-lines, Phil. Mag. xvni, pp. 294-8, 1859; and by Clerk Maxwell, On hills and 

 dales, ibid, xl, pp. 421-7, 1870. 



