166 THE GEEM THEORY OF DISEASE. 



of the upper incisors. In case of the wisdom teeth, the fact 

 that they are very often decayed before they are fully through 

 the gum is especially remarked ; and as a rule, if these decays 

 are carefully noted at a very early period of their progress, 

 it will be seen that they are different from other decays in 

 several respects. It always has its beginning under the free 

 margin of the gum. There is usually no change whatever 

 in the appearance of the tooth; the eye discovers nothing. 

 The surface seems normal, or, at most, the portion of the 

 tooth appears rather whitish ; but on trial with the excavator 

 the instrument will, apparently, break in through the enamel 

 prisms, disclosing a cavity of very slight depth. It often 

 happens that the enamel may be easily scraped away over a 

 considerable space, as though it was so much chalk. The 

 depth will present much variation ; often it is only a part of 

 the thickness of the enamel, at other times we may find it 

 extending into the dentine, in which it forms a veritable 

 cavity. If there is much depth, however, the characteristics 

 will have assumed the more usual type. 



Occasionally we see this character of decay (if it may be so 

 called) in the grinding surfaces of the wisdom teeth ; occa- 

 sionally in the first molar also, where the tooth has come 

 through very slowly, and the gum has been for a long time 

 in a state of chronic irritation. It is characteristic of this 

 effect that it is as often seen on the smooth surfaces of the 

 teeth as in the pits and grooves. No imperfection is necessary 

 to prepare the way for this manifestation. 



As we have said, this takes place under the gum ; is covered 

 by the gum. Now, as the tooth rises higher and the surface 

 thus affected becomes exposed, these spots are prone to become 

 the seat of true caries, with all of its usual manifestations. 

 However, very often caries does not take place. In this case 

 a whitish spot is seen, which gradually assumes a yellowish 



