166 PHILOSOPHY QJ? ZOOLOGY. 



of different layers of delicate fibres intimately united, exhi- 

 biting remarkable peculiarities in different species. It is 

 susceptible of a good polish, and is extensively used in the 

 arts. It is principally obtained from the different species 

 of elephant and hippopotamus. Some teeth, as the tusks 

 of these species, are entirely composed of ivory ; while, in 

 other species, it is intermixed with the enamel or cement* 



The Enamel is still harder than the ivory ; contains less 

 animal matter ; and consists of minute fibres, uniformly ar- 

 ranged perpendicular to its surface. It never occurs alone, 

 but always as a coating to the ivory. 



The Cement is of a softer substance than any of the pre- 

 ceding. It contains more animal matter, and is nearly si- 

 milar to ordinary bone. It occurs as a coating to the ivory 

 and enamel, or filling the interstices of their folds. 



When a tooth, consisting of these three substances, is 

 subjected to the action of the fire, the cement blackens first, 

 and then the ivory, while the enamel retains a good deal of 

 its original whiteness. These effects result from the char- 

 ring of the animal matter *. 



2. Formation. The teeth are formed in sockets in the 

 jaw-bones, which are termed Alveoli. In each socket, there 

 is a pulpy substance, consisting of gelatine and albumen, and 

 liberally supplied with bloodvessels and nerves. This pulp 

 is surrounded by a capsule, to which, however, it does not 

 adhere, except at the base. This capsule enters inio all 

 its sinuosities. From the pulp the ivory is secreted, the 



* On the surface of the tooth, there is sometimes deposited a substance 

 termed the Tartar of the teeth. It frequently assumes a yellow colour with 

 a smooth surface in the ox and the sheep, and has been ignorantly considered 

 as gold derived from the pasture. It is merely a precipitation from the sa- 

 liva. BERZELIUS found it to consist of earthy phosphate, 79.0; mucus, 

 not yet decomposed, 12.5; peculiar salivary matter, 1.0 ; and animal matter, 



soluble in muriatic acid, 7.5 100.0 An. Phil. voL ii. p. 38L 



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