THE TEETH. 49 



§ 139. 



The Enamel, substantia vitrea, forms a continuous layer 

 investing the crown of the tooth ; it is thickest upon the 

 masticating surface, and gradually diminishes towards the roots 

 until at last it terminates by a sharply denned or sometimes, 

 slightly-notched, border, ceasing sooner upon the contiguous 

 surfaces of the crowns, than upon their inner and outer sides. 

 The external surface of the enamel appears smooth, but is 

 always marked by delicate, close, transverse ridges, among 

 which more marked circular elevations may occur. 1 A delicate 

 membrane, discovered by Nasmyth, and which I will denominate 

 the cuticle of the enamel, [Nasmyth 3 s membrane, Eds.] 2 entirely 



causes, not only from those pointed out by Czermak — curvings and local enlarge- 

 ments of the canals and interlobular , spaces, but also from* a difference in density 

 without alteration of structure; and he states as a general law, that the curves of the 

 contour markings are in proportion to the primary curves of the dentinal tuhes at any 

 particular spot, and cross them at right angles. No markings are more divergent than 

 the outline of the tooth, and passing from within outward, they abut in succession upon 

 the external surface of the dentine, under the enamel and crusta petrosa, in the form 

 of granular patches. The outer extremities of these patches look like white rings on 

 the surface of the tooth. They are composed of coarse globular dentine, and gradually 

 thin out internally into mere streaks. When a tooth is macerated in acid, it may be 

 broken up into cones (triangles in section), as Dr. Sharpey first indicated, formed 

 by the normal dentine between the contour markings. In transverse sections, the 

 cones become, of course, rings. Finally, Mr. Salter points out that the enamel is 

 almost always imperfect opposite the " patches" at the outer ends of the contour 

 lines. — Eds.] 



1 [Czermak (1. c. pp. 4, 5) states that the fine regular annular ridges and furrows upon 

 the surface of the enamel, characterise the permanent teeth, and are not present upon 

 the enamel of the milk set. The ridges are closest at the margin of the crown, and 

 most distant towards its centre, where they finally disappear. In the space of a line, 

 there were, at the margin of the enamel, 84 — 72 ridges ; more internally, 36 — 30 ; 

 and where they began to be indistinct, only 18 — 12. — Eds.] 



2 [We have ventured to substitute the name " Nasmyth's membrane," for that of 

 the "cuticle of the enamel," used by Professor Kolliker, inasmuch as the latter term 

 gives a false idea of the relations of this important structure, which is much more 

 than a mere "cuticle of the enamel," and is in fact, as one of us has already shown, 

 (Huxley, On the Development of the Teeth, ' Quarterly Journal of Mic. Science,' 

 vol. I, p. 149, 1853), the calcified membrana preformativa of the whole pulp. 



This structure was first described, in its true relation to the dental tissues, by Mr. 



Nasmyth, in a memoir read before the Medical and Chirurgical Society, in January, 



1839, and which, illustrated with very good figures, was published in the twenty-second 



volume of the Society's Transactions (p. 310 — 328). Mr. Nasmyth states, that his 



ii. 4 



