168 PHILOSOPHY OF ZOOLOGY. 



tortuous bands of enamel did not become ossified, as the space 

 which it occupied would be left empty by its absorption or 

 decay. Instead of this, we find the place of the membrane 

 occupied by this osseous cement. In some cases, the ex- 

 ternal surface of this membrane encloses spaces which are 

 not filled up, as in the case of the horse. This view of the 

 matter will be confirmed by a view of the position of this 

 cement, wherever it occurs *. 



While the crown of the tooth is forming, the capsule is 

 covered with the gum. Towards the termination of the 

 process, however, the crown bursts through the gum, and 

 is elevated to its proper place, by the farther growth of the 

 body and fangs. 



The teeth of the mammalia exhibit very remarkable dif- 

 ferences with regard to their duration. The Milk Teeth 

 are produced in early life, and fall out before the animal 

 reaches maturity. These are replaced by others, which 

 continue during the vigour of the animal to old age. Some 

 of these Permanent Teeth are not preceded by temporary 

 ones, as the back teeth in the jaw, but occupy those spaces 

 which have been produced by the growth and lengthening 

 of the jaw. These castings and renewal of the teeth take 

 place at different times, according to the species. In the 

 elephant, wild boar, and a few other species, the grinding 

 teeth are shed and renewed in succession, throughout the 

 life of the animal. While the last formed tooth is wearing 

 down, another, of a larger size, and suited to the increase 

 of the jaw, is forming beneath, to supply its place. These 



* Sir EVERARD HOME, when speaking of this cement, says, " The liga- 

 njentous structure on which the third substance is formed, is divisible into 

 layers, shewing that it is made up of separate membranes ; and, between 

 these, small ossifications, in different places, are readily detected." Comp, 

 Anat. vol. i, p. 201, 



