CROWDED TEETH. 15 



through the diminution of the jaw by disuse. 1 

 But the teeth which are the most frequently over- 

 crowded are the lower incisors. The upper incisors 

 are less overcrowded, being commonly pressed 

 outwards by the lower arc of teeth fitting inside 

 them in biting. The lower incisors are correspond- 

 ingly pressed inwards and closer together. Yet the 

 upper incisors decay or at least are extracted 

 about twenty times as frequently as the closely 

 packed lower incisors. 2 Surely this must indicate 

 that the cause of decay is not overcrowding. 



1 P. 13 ; and Nineteenth Century, February, 1888, p. 21 1. 



2 Tomes's Dental Surgery, pp. 273-275. Tomes observes that 

 it is as yet uncertain in what way civilization predisposes to caries. 

 But he shows that caries is caused by the lime salts in the teeth 

 being attacked by acids from decomposing food in crevices, from 

 artificial drink such as cyder, from sugar, from medicine, and from 

 vitiated secretions of the mouth. It is evident that in civilized 

 races natural selection cannot so rigorously insist on sound teeth, 

 sound constitutions, and protective alkaline saliva. The reaction of 

 the civilized mouth is often acid, especially when the system is dis- 

 ordered by dyspepsia or other diseases or forms of ill-health common 

 under civilization. The main supply of saliva, which is poured 

 from the cheeks opposite the upper molars, is often acid when in 



