DENTITION. 117 



represents the deposite of crusta petrosa, as performed by a 

 third membrane, wholly distinct from the two others, and 

 exterior to them all, although it follows them in all their 

 folds. In the horse and the ox, the projecting processes of 

 the pulp, have more of a conical form, with undulating 

 sides; and hence the waved appearance presented by the 

 enamel, on making sections of the teeth of these animals. 



The tusks of the elephant are composed of ivory, and are 

 formed precisely in the same manner as the simple conical 

 teeth already described, excepting that there is no outer cap- 

 sule, and therefore no outer crust of enamel. The whole of 

 the substance of the tusk is constructed by successive depo- 

 sites of layers, having a conical shape, from the pulp which 

 occupies the axis of the growing tusk; just as happens in the 

 formation of a univalve shell which is not turbinated, as, for 

 instance, the patella. Hence, any foreign substance, a bul- 

 let, 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 pulp, as the growth of the tusk advances, re- 

 tires in proportion as its place is occupied by the fresh de- 

 posites of ivory. 



The young animal requires teeth long before it has attained 

 its full stature; and these teeth must be formed of dimen- 

 sions adapted to that of the jaw, while it is yet of small size. 

 But, as the jaw enlarges, 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 di- 

 mensions, 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 en- 

 larges, the second set of teeth gradually acquire their full 

 dimensions, and then, by their outward pressure, occasion 



