6 A MANUAL OF DENTAL ANATOMY. 



By this admirable arrangement no two teeth oppose each 

 other only, but each tooth in closure of the jaw impinges 

 upon two, so that should a tooth be lost, or even two alter- 

 nate teeth, still the corresponding teeth of the opposite jaw 

 are to some extent opposed, and thus remain useful. For 

 when a tooth is wholly unopposed, a process is apt to be 

 set up in the jaw by which the useless organ is gradually 

 ejected. The direction of the teeth in the upper is verti- 

 cally downwards and slightly forwards, while those of the 

 lower jaw are placed vertically, the molars tending slightly 

 inwards. 



It is usual to represent the dentition of any animal by 

 what is termed a dental formula, which enables the reader 

 at a glance to see the number of teeth of each variety pos- 

 sessed by the creature. Vthus, instead of writing out at 

 length that man has two inc\sors on each side in both upper 

 and lower jaws, one canine, tw> bicuspids or premolars, &c., 

 it is written thus : \ / SHld 1>O? 



T 2 1 z\ 3 



I. c. prm. m 



9 1 9 \ ^ 



or in the deciduous set : \ 



I. 2 c. I dm. - 20. 

 21 2 A 



For the purpose of description three parts of the tooth, , 

 are distinguished by name, viz., the crown, neck, and root.)V 



This distinction of parts which we make in describing 

 human teeth, when we speak of crown, neck, and root, is 

 applicable to the great majority of mammalian teeth, 

 though there are some few simple forms of teeth in which 

 no such differentiation of parts can be seen. 



The crown is that portion which is exposed above the 

 borders of the gum, and is in human teeth coated with 

 enamel ; the neck is that portion which corresponds to the 



