DENTITION. 157 



their surfaces next to the pulp, till the whole has 

 formed the first, or outer layer of ivory : in the 

 mean time, the inner surface of the capsule, 

 which is in immediate contact with this layer, 

 secretes the substance that is to compose the 

 enamel, and deposits it in layers on the surface 

 of the ivory. This double operation proceeds 

 step by step ; fresh layers of ivory being depo- 

 sited, and building up the body of the tooth, 

 and in the same proportion encroaching upon 

 the cavity occupied by the pulp, which retires 

 before it, until it is shrunk into a small compass, 

 and fills only the small cavity which remains in 

 the centre of the tooth. The ivory has by this 

 time received from the capsule a complete coat- 

 ing of enamel, which constitutes the whole outer 

 surface of the crown ; after which no more is 

 deposited ; and the function of the capsule 

 having ceased, it shrivels and disappears. But 

 the formation of ivory still continuing at the part 

 most remote from the crown, the fangs are gra- 

 dually formed by a similar process from the 

 pulp ; and a pressure being thereby directed 

 against the bone of the socket, at the part where 

 it is the thinnest, that portion of the jaw is ab- 

 sorbed, and the progress of the tooth is only 

 resisted by the gum; and the gum, in its turn, 

 soon yielding to the increasing pressure, the 

 tooth cuts its way to the surface. This process 

 of successive deposition is beautifully illustrated 



