THE TEETH 25 



The wall of the dental cavity or sac ultimately becomes the 

 periosteum of the tooth, developed from its deep layer after the manner 

 of bone formation, and deposited on the enamel or dentine. 



The next step is the rupture of the outer wall of the follicle, and 

 when the tooth becomes sufficiently hardened by the deposition of salts 

 in the different layers, the pressure from below caused by the increase in 

 size of the fangs, assisted by the pressure from without by the frequent 

 application of the upper and lower gums to one another, causes the 

 eruption of the tooth. 



The walls of the alveoli are completed by the ossification of the 

 septa which originally separated the follicles from one another. 

 Although the tooth may now be able to carry out its functions, growth 

 continues, new layers of dentine being constantly deposited on the old 

 layers from the dental pulp, with the result that the pulp cavity and 

 also the pulp gradually diminish in size and ultimately disappear. 



For a more detailed account of the development of the teeth, a 

 text-book on Embryology should be consulted. 



Temporary and Permanent Teeth 

 Two sets of teeth are developed, namely, temporary or milk teeth, 

 and permanent or persistent teeth. 



In each jaw of the adult male there are developed twelve 

 molars, six incisors, two canines, and occasionally one or two of the so- 

 called wolf's teeth. In the temporary dentition, however, there are six 

 incisors and only six molars — the first three on either side. A peculiar 

 feature in connection with the teeth is that as the table of the tooth 

 becomes worn out by friction, the alveolar cavity becomes gradually 

 filled up, so that the tooth becomes slowly pushed out from its socket. 

 This goes on throughout life, and thus we have, at successive periods of 

 the animal's existence, at first the crown, next the neck, and lastly the 

 fang actually in wear. Although in an aged animal the incisors appear 

 to be very long owing to the increase in length of their visible portion. 



