INTRODUCTION 



IN a work on the microscopic anatomy of the teeth it 

 may perhaps be considered necessary to define exactly what 

 is meant by a tooth. This might seem at first sight an easy 

 matter, but in formulating any exact definition we meet 

 with many difficulties. 



It may be said that if we except the bill of the bird, all 

 horny or calcified organs which occupy the commencement 

 of the alimentary canal may be looked upon as teeth, and 

 that their primary function is prehension. But such a defini- 

 tion does not cover the whole ground, for some teeth never 

 erupt, as the incisors of the female Narwhal ; teeth are 

 found in ovarian cysts, while the placoid scales of the 

 Sharks are identical in structure with true teeth. 



Although the bill of the bird does not appear to fall 

 under the category of teeth, birds may be looked upon as 

 modified reptiles, and there is little doubt that they are 

 descended from forms which possessed true teeth, but these 

 were lost before Tertiary times. 



The fossil birds described by Marsh were furnished with 

 teeth, and the Archseopteryx was a toothed animal with 

 strong affinities to both birds and reptiles. Among recent 

 birds Carl Rose has described the formation of a tooth-band 

 in a tern (Sterna Wilsoni), although it is quite rudimentary," 

 and no teeth are derived from it (6). 



Although the primary function of teeth is prehension, 

 they are also used in many instances as weapons of offence 

 and defence, as the tusks of the Boar ; and those that are 

 developed exclusively in the male serve as sexual weapons. 

 The enormous canines of the Walrus assist in locomotion 

 and grasping, as well as in combat, and the greatly developed 

 incisors forming the tusks of the Elephant are used in 

 uprooting trees and carrying heavy weights, while the Beaver 

 employs its sharp incisors in felling timber to form its dams. 

 In the human being teeth serve as aids to speech. 



These organs are variously adapted by their form and 



B2 



