ENAMEL. 



371 



Fig, 234.' 



and inside of the bodies of the teeth, and thinnest on the surfaces 

 where adjoining teeth come into contact. It forms ridges on the 

 surface and free borders of the incisor teeth of the permanent set 

 only. 



Enamel consists of solid fibres of a prismatic form, ^m^ marked 

 by transverse lines^whose length determines the thickness of the 

 enamel. (Fig. 234, A.) Their 

 inner extremity abuts upon the 

 dentine ; their general direction 

 being nearly at right angles to 

 the surface of the latter. In 

 form they are hexagonal prisms, 

 slightly undulating, and from 



gfl'os to 5753 of an incn in dia ' 

 meter. Hence the external sur- 

 face of the enamel presenting 

 the hexagonal extremities of the 

 fibres, resembles the simple sca- 

 ly epithelium. (Fig. 234, B.) A 

 delicate membrane covers the 

 external surface of the enamel, 

 named "Nasmyth's membrane," from its discoverer. This is so 

 closely united with the latter that its existence can be demon- 

 strated only by the action of hydrochloric acid on the subjacent 

 enamel. It is a calcified 1 simple membrane, only about TFTST f an 

 inch thick. It is distinguished, however, by the great resistance it 

 offers to chemical reagents, and its consequent appropriateness as a 

 protection to the bodies of the teeth. Boiling water, concentrated 

 acetic acid, hydrochloric and sulphuric acids, have no effect upon 

 it; and nitric acid only renders it yellow. Nor is it changed by 

 the caustic alkalies. 



The assertion by Retzius that a similar membrane exists between 

 the inner surface of the enamel and the dentine, is probably incor- 

 rect. 



Though the enamel-fibres are so firmly united, no intermediate 

 substance can be discovered. Nor does Kolliker find the canals 

 mentioned by Todd and Bowman between the enamel-fibres. Clefts 

 are, however, often seen between them, especially in the middle of 



A. Vertical section of enamel, showing the stri 

 of the fibres. B. Enamel fibres seen endwise. 

 Magnified 3.50 diameters. (Rettius.) 



1 Dr. Huxley maintains that this is the calcified membrana praeformativa of the 

 whole pulp. Quarterly Journal of Microscopic Science, vol. i. p. 149. 



