DENTITION. 143 



calcareous phosphate in the cells of the pulp which 

 occupies the axis of the growing tusk. Hence any 

 foreign substance, a bullet, for example, which may 

 happen to get within the cavity occupied by the 

 pulp, becomes, in process of time, encrusted with 

 ivory, and remains embedded in the solid substance 

 of the tusk. The unossified portion of the pulp, 

 becoming smaller in size, as the osseous part of the 

 tusk increases in its dimensions, occupies only its 

 lower and central part. 



The young animal requires teeth long before it 

 has attained its full stature ; and these teeth must 

 be formed of dimensions adapted to that of the jaw, 

 while it is yet of small size. But as the jaw en- 

 larges, and the teeth it contains admit not of any 

 corresponding increase, it becomes necessary that 

 they should be shed, to make room for others of 

 larger dimensions, formed in a more capacious 

 mould. Provision is made for this necessary 

 change at a very early period of the growth of the 

 embryo. The rudiments of the human teeth begin 

 to form four or five months before birth : they are 

 contained in the same sockets with the temporary 

 teeth, the capsules of both being connected together. 

 As the jaw enlarges, the second set of teeth gradu- 

 ally acquire their full dimensions ; and then, by 

 their outward pressure, occasion the absorption of 

 the fangs of the temporary teeth, and, pushing them 

 out, occupy their places.* 



As the jaw-bone, during its growth, extends 



* It is stated by Rousseau that the shedding of the first molar 

 tooth both of the Guinea-pig, and the Capibara, and its replace- 

 ment by the permanent tooth, take place a few days before birth. 

 (Anatomie Comparee du Systeme Dentaire, p. 164.) 



