THE TEETH. 477 



united with, the enamel, that it can be demonstrated only by the use of 

 hydrochloric acid. According to Berzelius and Retzius,* a similar mem- 

 brane exists between the internal usually irregular surface of the enamel, 

 and the dentine, but I have been unable to find it. The enamel is bluish, 

 transparent in thin sections and much more brittle and harder than the 

 other dental structures, so that it is hardly touched by the knife, and 

 yields sparks with steel (Nasmyth). Chemically, it may be regarded as 

 osseous substance, containing the smallest possible proportion of organic 

 matter ; but whether the latter belong to the class of collagenous sub- 

 stances or not is uncertain. According to Von Bibra, the enamel 

 contains: 



From a Molar of a From a Molar of 



Woman 25 years of age. an adult Man. 



Phosphate of lime, with some fluoride of cal- 

 cium, 81-63 



Carbonate of lime, 888 



Phosphate of Magnesia, .... 2*55 



Salts, 0-97 



Cartilage, 5'97 



Fat, a trace 



100-00 10000 



Organic matters, 5'97 3 59 



Inorganic matters, 9403 9651 



The enamel, as its fibrous structure indicates, consists entirely of the 

 so-called prisms or fibres of the enamel (Fig. 189) ; long, solid prisms 



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 endeavored 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. TRS.] 



* [Berzelius and Retzius obviously saw Nasmyth's membrane (Retzius, in Mail. Archiv, 

 1837, pp. 53, 54). The latter says, that 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 dis- 

 solved, a membrane swimming in the fluid, " I examined it immediately under a conside- 

 rable magnifying power; it appeared to be pierced by a multitude of closely arranged little 

 holes, but exhibited no trace of fibres." But he states expressly, that this membrane was 

 "internal to the enamel fibres, as Berzelius clearly points out," and therefore failed to recog- 

 nize its true relations with the enamel. TRS.] 



