436 CRANIA FROM THE MOUNDS OF FLORIDA. 



that the disposition for a tooth to be twisted is an expression of its inability to 

 resist a force coming to it from without, and that this force resides in the lower 

 jaw and is a result of its motion in mastication. — In the lower jaw the premolars 

 are apt to undergo torsion quite distinct from that seen in the first and second upper 

 molars. The first premolar is inclined forward and the second premolar backward. 

 This disposition, as already stated, places the first premolar in harmony with the 

 canine and incisors, and the second premolar in harmony with the molars. The 

 lower premolars may be said to occupy a position between two distinct sets 

 of teeth which are functionally active, those at the anterior part of the dental 

 arch and those at the sides far back. In the mammalian orders these teeth are 

 exceedingly variable in number and form, but the rule above given is quite con- 

 stantly adhered to, and even in the human jaw where there are but two teeth of 

 the premolar series, it is found that they do not work together so much, as the first 

 assists the canine and the second the first molar. 



Mastication tends to displace the teeth from their alignments in a variety of ways. 

 In the upper jaw the attrition pushes the incisors forward and the molars outward; 

 and in the lower jaw all the teeth inward. The least amount of deviation is noted 

 in the third molars of both upper and lower jaws. 



A casual examination will show that the teeth of the upper jaw "wear," for 

 the most part, on the palatal aspect. It is far different in the teeth of the lower 

 jaw. Here the incisors, canine and the first premolar wear as a rule across the 

 crown, the second premolar, first and second molar wear on the buccal part of the 

 crown, while the third molar wears as do the incisors, canine and first premolar. — 

 In middle life a disposition exists for the teeth to be pushed out of their sockets. 

 This is notably the case in skulls Nos. 1,781 and 1,782. With this tendency an 

 exaggeration of the molar-inclinations above named is noted, and the attrition-effects 

 are especially marked, — namely, on the palatal surface of the maxillary and on the 

 buccal aspect of the mandibular molars. In No. 1,781 the teeth are worn but are 

 not displaced. 



In No. 1,782, by the constant imj)act on the palatal side the tooth is pushed 

 outward, the grinding surface being transferred from the face of the crown to 

 the palatal side of the crown and the neck. The disposition to outward inclination 

 of the molars is much greater in some individuals than in others. Broca uses the 

 term hyperbolic to express the shape of the hard palate as determined by the 

 degree of divergence of the sides of the dental arch (See Section 6). I believe 

 that the divergence of the arches is equivalent to the outward inclination of the 

 molars and, all things remaining the same, the greatest degree of divergence will 

 be found to correspond to the disposition for the teeth to wear on the palatal half 

 of the crown. 



In No. 1,782, as already noted, the second molar on both sides is turned inward. 

 The force of attrition on the palatal aspect of the cusps, while tending to force the 

 tooth toward the buccal surface, has, by wearing away the crown, so far disturbed the 

 equilibrium of the tooth in the jaw as to turn the crown in the direction of the grind- 



