The Head and Neck 83 



The Teeth 



STRUCTURE, DISPOSITION, and DEVELOPMENT. 



In the t5rpical mouth there are twenty permanent teeth in the 

 upper jaw and twenty-two in the lower. 



The structure of the incisors is noticeable for the arrangement 

 of three cusps of which the central is the largest. The four inner 

 incisors of both jaws have these well developed, but in the comer 

 incisors the middle cusp is very large and the lateral ones rudi- 

 mentary or absent. The base of the crown on the inner aspect is 

 marked by a ledge which is extended to each cusp as a ridge or 

 cingulum. 



The canines are large and firmly and deeply embedded in the 

 maxillary bone immediately behind the maxillo-premaxillary suture. 

 Their pulp cavity is capacious and extends almost the entire length 

 of the tooth. 



Posterior to the canines are four teeth which were designated 

 by Owen as premolars, owing to deciduous or milk teeth being dis- 

 placed by them. As a matter of fact, the first of this group, count- 

 ing from before backward, has more the character of a true molar, 

 it having only a germinal deciduous predecessor, which seldom be- 

 comes calcified and generally disappears before birth, after being 

 transitorily manifested in the papillary stage. It is the smallest of 

 the four and has a single root and well-defined neck. The second 

 and third premolars resemble one another closely, are considerably 

 larger than the first, and possess two roots, of which the posterior 

 is the larger. The three anterior premolars are not in as close con- 

 tact as the posterior teeth, being separated by slight intervals. The 

 fourth premolar is the largest and strongest of the premolar series. 

 It is known as the superior sectorial. It is implanted by three roots, 

 two external and one internal. The crown is composed of two main 

 lobes supported by the external roots and a small one supported by 

 the internal root. The premolars of the lower jaw are similarly 

 arranged with the exception of the fourth which corresponds in po- 

 sition, approximate size, and number of roots, to the third premolar 

 of the upper jaw- 



The true molars of the upper jaw are two in number and 

 are situated behind the premolars. They are permanent from the 

 the outset and do not displace milk teeth. They possess three roots. 



