474 STEUCTUEE AND DEVELOPMENT OF TEETH, W. WALDEYER. 



in polarised light are replete with interest. According to these, fully 

 developed enamel exhibits strongly negative double refraction, and is 

 probably uniaxial ; whilst young enamel presents positive double re- 

 fraction. Adult enamel becomes positive on being exposed to a tempe- 

 rature of 800 C. Hoppe-Seyler (69), in one of his analyses, found the 

 composition of the enamel of the newly born infant to be P0 5 3 Ca 

 = 75-23, C O a , Ca = 7-18, Cl Ca = 0-23, P0 5 3 Mg = 1-72. 

 Organic compounds = 15'59. The enamel of adults contains only 

 from one to three per cent, of organic constituents ; but, on the other 

 hand a large quantity of phosphate of lime. A remarkable feature is 

 the presence of a small proportion of fluorine, 



THE CTJTICULA (persistent capsule of Nasmyth, 22 ; schmel- 

 zoberhautchen of Kolliker) forms an extremely resistant invest- 

 ment not more than 1 2^ in thickness, covering the exposed 

 portion of the teeth, and disappearing wholly when they are 

 mature. When the enamel is present, the under surface fre- 

 quently presents the impression of prisms in the form of small 

 square areas. 



Kolliker and others more recently have improperly applied the term 

 enamel membrane to the cuticula, since it is developed with equal 

 distinctness in teeth in which the enamel is absent, as for instance in 

 the Pike. 



In young teeth, examined when in the act of perforating the 

 gum, the cuticula may be easily detached as a whole after 

 slight action of hydrochloric acid. It may then be tinted with 

 solution of nitrate of silver, which causes the appearance of 

 figures similar to large epithelial cells. These, as the history 

 of the development of the teeth shows (see this), are the corni- 

 fied cells of the so-called external epithelium of the enamel 

 organ, from which the cuticula is formed. 



The chemical relations of the cuticula dentis indicate that it 

 belongs to the category of horny substances. According to the 

 statements of Kolliker (58), which I am able to corroborate, 

 boiling water and mineral acids exert no action upon it, except 

 that it is stained of a yellow colour by nitric acid. When 

 boiled with caustic potash or soda, it softens, and when burnt 

 yields a smell resembling that of horn. I have not been able to 

 prove the presence of lime in the cuticle of man ; small traces of 



