vii.] THE EXTERNAL SKELETON. 251 



absorption of the fang of its predecessor. Replacement is 

 not the effect of mechanical pressure, but the two actions 

 (development and absorption) proceed harmoniously, the now 

 fangless milk-tooth easily falling out when its successor is 

 ready to take its place, unless through some abnormality 

 of growth the aid of the dentist has become necessary. 



Before mentioning the order of succession, the teeth them- 

 selves must be shortly described. 



1 8. The TEETH OF MAN, when adult, should be thirty-two 

 in number. 



As the two sides of each jaw are alike, it will be suffi' 

 cient to notice the eight teeth above and eight below of 

 each side. 



The fangs or roots of all teeth are firmly fitted into corre-. 

 spondingly shaped cavities in the bones of the jaws, which 

 cavities are termed alveoli, and 

 this mode of union of parts (like a 

 nail driven into any substance) is 

 termed gontphosis? 



The front tooth of each lateral 

 half of the upper jaw has a chisel- 

 shaped crown with a horizontal 

 cutting edge. It has a single, long, 

 tapering fang. 



The second tooth is like the first, FlG 210 ._ UppER AND LowEK 



and these two, on account of their TEETH OF LEFT SIDE OF AN 



cutting shape, are called incisors ; ADULT MAN. 



but we Shall See that there is another ', incisors ; c, canines ; /;, pre- 



reason why they belong to a special lrs ; '"' m< 



category. 



The third tooth above is more pointed, more conical, and 

 has a longer though still single fang. It is called the eye- 

 tooth, or canine. 



The fourth tooth above has a flatter and broader (from 

 within outwards) crown, and bears two pointed tubercles (or 

 cusps), one internal, the other external. Its fang is flattened 

 and vertically grooved, showing a tendency to division, while 

 at its end it is in general actually divided and has two 

 apertures, one in each division. 



The fifth upper tooth is like that last described, and the 

 two, from the number of their tubercles, are called bicuspid, 

 and from their more flattened crowns (better adapted for 

 grinding) they are also called molars. The fifth differs from 



1 From f6/ji(poc, a nail. 



