TEETH. 245 



3. The enamel. It covers the crown of the tooth as far as 

 the neck. It is very hard, and is obviously intended to prevent 

 the tooth from wearing so fast as it otherwise would do while 

 performing the office of mastication. The enamel has no carti- 

 lage, and, consequently, has a higher specific gravity than the 

 ivory of the tooth. 



4. The capsule. This is a thin double membrane which, be- 

 fore extrusion, covered the whole tooth. It is gradually worn 

 away on the crown ; though Mr Nasmyth has frequently found 

 it either entire, or fragments of it on the crown even of an 

 adult tooth.* It remains during life on the roots of the teeth. 

 But it is frequently ossified, and then gets the name of crusta 

 petrosa. 



Leuwenhoek first observed in 1678 that the body of the tooth 

 is composed of a congeries of transparent tubes, so small, that six 

 or seven hundred of them together do not exceed the size of a hu- 

 man hair.f Purkinje, in his work on the teeth, published in 1835, 

 confirmed this observation of Leuwenhoek. If the calcareous 

 salts be removed by steeping a tooth in dilute muriatic acid, and 

 the cartilage be examined under a sufficiently powerful micro- 

 scope, it is found, he says, to consist of transparent tubes, running 

 from the centre to the circumference. They are not straight, 

 but curved, and their diameter does not exceed y-g-o th of an Eng- 

 lish line. They become smaller as they reach the outer surface 

 of the tooth, and seem to terminate in cells. They send out nu- 

 merous branches, especially towards their external extremity. 

 These tubes, according to Muller, in the tooth, not acted on by 

 muriatic acid, are white and opaque, being filled with the calca- 

 reous salts of bone ; not in crystals, but in very fine powder 

 usually cohering together. The ivory, it would appear from 

 Retzius's observations, is deposited layer by layer round the sur- 

 face of the pulp ; the most external layer having been first de- 

 posited. 



The enamel adheres internally to a thin membrane, which 

 long resists water. It consists of hexagonal tubes which proceed 

 from the membrane. J 



* On the structure, physiology, and pathology of the tooth. Medico- Chirurgi- 

 cal Transactions, Vol. xxii. 



f Phil. Trans, xii. 1002. 



| On the structure of the teeth, the reader is referred to an elaborate paper 

 by Retzius, published in the Memoirs of the Stockholm Academy for 1836, and 



