476 SPECIAL HISTOLOGY. 



139. The Enamel, substantia vitrea, forms a continuous layer 

 investing the crown of the tooth ; it is thickest upon the masticating sur- 

 face, and gradually diminishes towards the roots until at last it termi- 

 nates by a sharply-defined 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.* A delicate membrane, 

 discovered by Nasmyth, and which I will denominate the cuticle of the 

 enamel [_NasmytJis membrane, TttS.],f entirely covers, but is so closely 



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 al- 

 ways imperfect opposite the "patches 1 ' at the outer ends of the contour lines. TRS/] 



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

 surface of the enamel, characterize 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. TRS.] 



t [We have ventured to substitute the name " Nasmyth's membrane," for that of the "cuti- 

 cle 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 So- 

 ciety's Transactions (p. 310-328). Mr. Nasmyth states, that his attention had been drawn to 

 fragments of a membrane which he found floating in the acid in which teeth had been 

 macerated ; " after a minute and careful examination, however, I was able to demonstrate 

 with the greatest certainty, that they were derived from the external surface of the enamel, 

 and that they were continuous with the structure covering the fang, which latter is itself 

 continued into the chamber of the tooth. 1 afterwards succeeded in tracing this covering on 

 the whole surface of the enamel and fang of the tooth in one continuous envelop ; and 

 eventually, I was enabled to remove it from the crown of the tooth in the form of a distinct 

 coat or capsule ; this covering, which I proved to exist externally to the enamel, I have 

 termed 'the persistent dental capsule,'" p. 312. 



"In all cases where this covering has been removed by means of acid, it has, of course, 

 the appearance of a simple membrane, in consequence of the earthy deposits having been 

 dissolved, and of there being only present the animal tissue. The structure and appearance 

 of the covering detached in this manner from the enamel, are the same in every respect as 

 those observed in the capsule of the unextruded tooth ; consisting, like it, of two layers, fibrous 

 externally, and having on its internal surface the peculiar reticulated appearance common 

 to both, and shown at Plate V. Fig. 6," p. 313. 



"On examining carefully fine sections of .several teeth under the microscope, I perceived 

 here also, that the structure in question was continuous with the crusta petrosaof the fang of 

 the tooth," p. 313. 



