THE TEETH IN GENERAL 31 



temporo-mandibular articulation is a mere hinge, 

 admitting of opening and closing of the jaw with- 

 out any lateral motion. The jaw is short and 

 stout to sustain the force of hard biting, and the 

 teeth are developed vertically, with long points 

 and blades, in such a manner as to resist the 

 greatest strain. In the other extreme form — 

 the Herbivorous mammalia — the teeth are broad 

 and flat and we find the temporo-mandibular artic- 

 ulation is open to allow extreme lateral movement 

 of the mandible. The jaw bones are light and the 

 surfaces of the teeth are roughened to produce 

 a good grinding surface and there is less strain 

 on the jaws than in the carnivorous animals. In 

 the elephant we find the dental tissue arranged 

 in plates and the tooth is more effective by an 

 antero-posterior movement and the jaws and ar- 

 ticulation are arranged for that purpose. So in 

 all animals there is a close relation existing be- 

 tween the shapes of the teeth and the jaw move- 

 ments. 



Tooth Forms. The original and primitive 

 form of the tooth is that of the single, simple cone, 

 as illustrated in the teeth of fishes and reptiles, 

 which are simple cones with but little modifica- 

 tion. From this primitive form all other forms 

 have been derived by modification and duplica- 

 tions of the single cone. Thus the incisors of 

 man are formed of a single cone, the base of 



