50 SPECIAL HISTOLOGY. 



covers, but is so closely united with, the enamel, that it can be de- 

 monstrated only by the use of hydrochloric acid. According to 

 Berzelius and Retzius, 1 a similar membrane exists between the in- 

 ternal usually irregular surface of the enamel, and the dentine, 

 but I have been unable to find it. The enamel is bluish, trans- 

 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, how- 

 ever, 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. I 

 afterwards succeeded in tracing this covering on the whole surface of the enamel and 

 fang of the tooth in one continuous envelope ; and eventually, I was enabled to remove 

 it from the crown of the tooth in the form of a distinct coat or capsule; this cover- 

 ing, 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 pecu- 

 liar 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 

 petrosa of the fang of the tooth," p. 313. 



Mr. Nasmyth does not distinguish quite clearly in the text, between the proper 

 capsular membrane on the fang and the crusta petrosa itself, though his figures (PI. 

 5, figs. 4, 5.) exhibit the two structures as sufficiently distinct, and he hesitates, at p. 

 316, to decide what relation the outer layer of pale yellowish or brown substance in 

 the cement of the teeth of the Elk, Ox, Bradypus, &c, may have to the " persistent 

 capsule." 



We have not retained Mr. Nasmyth's own term for his discovery, because, as one of 

 us has endeavoured to show, (Huxley, 1. c.) while he accurately described its relations 

 to the other dental tissues, he mistook its true nature. (See Appendix, § 8, on the 

 Development of the Teeth.) But, on the other hand, as no one has, before or since, 

 distinctly described his " persistent capsule," we have thought it desirable, that his 

 name should be associated with the structure. — Eds.] 



1 [Berzelius and Retzius obviously saw Nasmyth's membrane (Retzius, in Mull. 

 Archiv, 1837,pp.53-54). The latter saysjthat on macerating a large piece of enamel from 

 the fossil tooth of a horse (dug out of a peat-moss) in dilute acid, he found after the 

 enamel was dissolved, a membrane swimming in the fluid, "I examined it immediately 

 under a considerable magnifying power ; it appeared to be pierced by a multitude of 

 closely arranged little holes, but exhibited no trace of fibres." But he states ex- 

 pressly, that this membrane was " internal to the enamel fibres, as Berzelius clearly 

 points out," and therefore failed to recognise its true relations with the enamel. — 

 Eds!] 



