494 SPECIAL PHYSIOLOGY. 



in the section on Development. The period of the cutting or eruption 

 of the temporary teeth is as follows: 



The milk teeth begin to appear at about the seventh month, and are 

 completed at the expiration of the second year; but considerable dif- 

 ference exists in regard to the precise periods of their eruption, fre- 

 quently the first teeth appearing as early as the fifth or sixth month, 

 and some infants being born with teeth. The annexed diagram shows 

 the usual order and average time at which the milk teeth are cut, the 

 numbers indicating month*. 



I I 



I 7 I 



C x "'*$/ ,,C 



M,. "M8/ M 



'"24-- 



The lower middle incisors appear first, and generally the lower teeth 

 are cut before the corresponding teeth of the upper jaw. Before the 

 cutting of the teeth, the edges of the jaw, previously sharp, hard, arid 

 pale, become rounded and swollen, and of a darker color, and the apex 

 of the future tooth appears, like a white line or spot, through the 

 gum. 



The milk teeth, having, for a time, fulfilled the office of mastication, 

 fall out, and are succeeded by the permanent set, destined to serve the 

 same purpose through the remainder of life. Teeth, once formed, 

 cannot increase in size. The mijk teeth, though sufficiently large for 

 the infantile jaws, and strong enough to resist the action of the less 

 powerful muscles working them against the softer food consumed in 

 the earlier periods of life, would not be strong enough for the fully 

 developed jaws and muscles, and the harder food, of the adult. Hence, 

 they are removed to make way for a larger set, which also, when once 

 formed, undergo no change in size. Their formation and classification 

 commence, indeed, at very early periods of life, the ossification of the 

 first permanent teeth beginning at the age of six months, and that of 

 the last molars, or wisdom teeth, at about twelve years of age; yet 

 their size is proportionate to the dimensions of the future alveoli and 

 jaws, and to the future wants of the still undeveloped adult. The for- 

 mation of the permanent teeth presents one of the clearest examples 

 of anticipative design in the animal economy; for they are laid down, 

 and their crowns even are fully formed, whilst the jaw itself is still too 

 small for their proper accommodation, and their future alveoli do not 

 even exist. 



The eruption of the permanent teeth corresponds, generally, with 

 that of the milk set. Thus, the permanent incisors succeed to the 

 temporary incisors, the canines of the one set, to those of the other, 

 and the two permanent bicuspids, to the two temporary molars. The 



