THE TEETH 



1123 



side) when present. The second lower molar has usually four cusps (75 to 85 per cent, of cases), 

 otherwise five, the fifth usually small or rudimentary. The roots are sometimes confluent. 

 The lower third or wisdom tooth, like the upper, is usually small and exceedingly variable 

 (fig. 875). It has usually four or five cusps; but the number may be increased to six or seven, 



Fig. 879. — -Model op Ectoderm of Jaw of Human Embryo 40 mm. Long, Showing 

 Dental Ridge with Enamel Organs for the First Teeth. (Kingsley, after Rose.) 



or reduced to'three, two, or one. The roots are often short and fused into a conical mass 

 in which sometimes only a single canal is present. 



The dental arches. — On comparing the upper and the lower dental arches, it is seen that 

 the upper (fig. 852) forms an elliptical curve, while the lower (fig. 864) resembles a parabola. 



Fig. 880. — Section Showing Later Stages of Tooth Development. (Szymonowicz.) 



-.Epithelium of oral cavity 



Enamel — '^- 

 pulp 5-^ 



Inner 



enamel 



cells 



-.4 



^1 



Neck of 

 enamel organ 



Dental ridge 

 of permanent 

 tooth 



Bone trabecu- 

 Iss of lower jaw 



Dental _, 

 papilla 



,.^« 



^'^l 



The upper arch is slightly larger (due chiefly to the slant of the teeth, as previously explained) 

 so that it slightly overlaps the lower when the teeth are in occlusion. Thus, as showii in fig. 

 876, the upper incisors (and canines) overlap the lower. The buccal cusps of the lower pre- 

 molars and molars fit into the groove between the upper buccal and lingual cusps; while the 

 upper Ungual cusps correspond to the groove between lower buccal and lingual cusps. This 

 arrangement favors a more perfect mastication (see fig. 877). 



Moreover, when viewed from the side (fig. 876), it is seen that in general, the corresponding 

 teeth of the upper and the lower arches are not opposite, but alternate with each other. This is 



