270 STUDIES IN ADVANCED PHYSIOLOGY. 



with reference to the stress to which they are subjected. 

 In a perfectly new tooth which has not suffered any abra- 

 sion the enamel is covered over with a slight cuticle, also 

 of skin origin, which, however, at once disappears when 

 the tooth is put to hard use. 



Hygiene. 



While in the dentine of a tooth a slight growth and re- 

 pair may be possible, it is not possible to remedy a defect 

 in the tooth when it assumes at all large dimensions, while 

 of course a cracking or loss of the enamel is at once irre- 

 parable. As the teeth are almost wholly of a mineral na- 

 ture they are subject to the action of acids, and as in the 

 decomposition of foods various organic acids are produced 

 there is a constant corroding action going on which may 

 finally destroy the teeth. While the enamel is over them, 

 this corroding action is largely prevented, but as soon as 

 the hard enamel gives way and the acids have direct access 

 to the softer dentine beneath, the corroding influence pro- 

 ceeds more rapidly. This explains in the clearest way the 

 necessity for absolute cleanliness of teeth in order to pre- 

 serve them. 



Around the teeth, sometimes even in spite of good care, 

 there is formed a deposit familiar as tartar. This is not wholly 

 derived from bits of food which have not been removed, but 

 the tartar is mainly a deposit of lime salts derived from the 

 saliva. In ordinary saliva lime salts are present, and as the 

 teeth are continually bathed in such a lime solution a de- 

 position of these salts around the teeth goes on and so gives 

 rise to the crusty tartar. This act of deposition is illus- 

 trated in the lime crusts which form on the inside of kettles 

 or boilers in which hard water has been continually kept. 

 When such tartar is merely a pure lime deposit it is pos- 

 sible that it may interfere in no way with the teeth, except, 

 possibly, to irritate the gums, when these are pressed down 

 on the tartar underneath them and so produce bleeding, but 

 most frequently there is deposited along with this tartar and 



