1556 



HUMAN ANATOMY. 



Fig. 1315. 



Dental arches seen from the side, showing relations 

 of upper and lower teeth. 



as those of the upper ones ; hence it follows that the alveolar arches of the upper and 

 lower teeth are in different curves, the latter having a great transverse distance 

 between the necks of the wisdom-teeth. 



The right half of the jaw is usually the stronger and the teeth form a smaller 

 curve. It has been pointed out in the section on the motions of the lower jaw that 

 the line between the molars, and probably the bicuspids, is a part of the circumference 

 of a circle the centre of which is near the top of the lachrymal bone ; it may now be 

 added that the line of the cutting edges of the lower incisors is a part of a transverse 

 curve with the convexity upward. There is no corresponding concavity in the line 

 of the edges of the upper incisors, for the lower do not naturally meet them ; but the 

 convexity plays along the lingual surfaces of the upper ones. The position and shape 

 of the superior incisors make their inner surface a part of a vault. A transverse 

 section of this is necessarily a curve with an upward convexity. The wearing of the 

 outer corners of the lateral incisors is evidence of this action. The fact that there is 



no purely lateral motion, but an oblique 

 one, modifies, without invalidating, this con- 

 ception. 



The relations of the roots of the su- 

 perior teeth to the antrum are very impor- 

 tant. The incisors have no relation with it 

 whatever. The long fang of the canine is 

 opposite the wall between the antrum and 

 nose, and separated by diploe from the 

 former. The first bicuspid is usually sepa- 

 rated in the same manner. The second is 

 very close to its front wall and may indent 

 the floor. The first and second molars 

 always do this. The wisdom-tooth also in- 

 dents it at the junction of the floor with the 

 posterior wall. Its relation, owing in part 

 to its varying development, is less certain. Exceptionally the first bicuspid and 

 even the canine may be in contact with the antrum. Thus caries of the roots of 

 any of the molars, but especially of the first and second, sometimes of the second 

 bicuspid and exceptionally of the first, or even of the canine, may lead to inflamma- 

 tion of the antrum. In certain cases pus may pass directly into it from the root. 



The Temporary Teeth. — In the first dentition the dental arches differ from 

 the permanent ones in showing a broader curve, more nearly approaching half a circle, 

 symmetrical on both sides, in having the upper incisors less slanting, and the molars 

 of each row more nearly vertical. This implies less difference in curve between the 

 jaws. The line of meeting of the teeth is more horizontal. The crowns increase in 

 size from the incisors backward. In the young child the antrum is but a small pouch, 

 and the roots of the first teeth and the sacs of the second lie in diploetic tissue. The 

 first permanent molar, as its fangs grow, is nearest the antrum, having extended above 

 it by the end of the second year. In its early stages the first bicuspid is too far forward 

 to have any relation to the antrum, and the second reaches only its extreme anterior 

 border. The second permanent molar is at first behind rather than below it, and the 

 third is still higher. As these descend they swing around the antrum. Thus the roots 

 of only the first permanent molar are in approximately the same relation to the antrum 

 throughout. 



DEVELOPMENT OF THE TEETH. 



About the beginning of the seventh week of fcetal life the ectoblastic epithelium 

 presents a thickening along the margins of the oral cavity. The ridge-like epithelial 

 proliferation, or labio-deyital strand, so formed grows into the surrounding mesoblast 

 and divides into two plates which, while still continuous at the surface, diverge almost 

 at right angles at the deeper plane. The lateral or outer plate is vertical, and cor- 

 responds to the plane of separation which soon occurs in the differentiation of the 

 borders of the lips and jaw. The median or inner plate grows more horizontally into 

 the mesoblast, and is the one intimately concerned in the tooth development ; for this 



