TEETH ERUPTION. 69 



socket by the progressive lengthening of the pulp, the deposition of 

 ivory, and the adhesion of the ivory to the contiguous portion of the 

 sac, the pressure of the socket causes the reflected portion of the sac 

 and the edge of the tooth to approach, and the latter to pass through 

 the gum. The sac has thereby resumed* its original follicular con- 

 dition, and has become continuous with the mucous membrane of the 

 mouth. The opened sac now begins to shorten more rapidly than the 

 fang lengthens, and the tooth is quickly drawn upwards by the con- 

 traction, leaving a space between the extremity of the unfinished 

 root and the bottom of the socket, in which the growth and comple- 

 tion of the fang is more speedily effected. 



During the changes which have here been described as taking 

 place among the dental sacs contained within the jaws, the septa 

 between the sacs, which at first were composed of spongy tissue, 

 soon became fibrous, and were afterwards formed of bone, which 

 was developed from the surface and proceeded by degrees more 

 deeply into the jaws, to constitute the alveoli. The sacs of the ten 

 anterior permanent teeth, at first enclosed in the submucous cellular 

 tissue of the deciduous dental sacs, and received during their growth 

 into crypts situated behind the deciduous teeth, advanced by degrees 

 beneath the fangs of those teeth, and became separated from them 

 by distinct osseous alveoli. The necks of the sacs of the permanent 

 teeth, by which they originally communicated with the mucous 

 lining of the secondary groove, still exist, in the form of minute ob- 

 literated cords, separated from the deciduous teeth by their alveolus, 

 but communicating through a minute osseous canal with the fibrous 

 tissue of the palate, immediately behind the corresponding deciduous 

 teeth. " These cords and foramina are not obliterated in the child," 

 says Mr. Goodsir, "either because the cords are to become useful 

 as ' gubernacula' and the canals as ' itinera dentium ;' or, much more 

 probably, in virtue of a law, which appears to be a general one in 

 the developement of animal bodies, viz : that parts, or organs, which, 

 have once acted an important part, however atrophied they may after- 

 wards become, yet never altogether disappear, so long as they do not 

 interfere with other parts or functions" 



Succession. The periods of appearance of the teeth are extremely 

 irregular ; it is necessary, therefore to have recourse to an average, 

 which, for the temporary teeth, may be stated as follows, the teeth 

 of the lower jaw preceding those of the upper by a short interval : 



7th month, two middle incisors. 18th month, canine. 



9th month, two lateral incisors. 24th month, two last molares. 



12th month, first molares. 



The periods for the permanent teeth are, 

 6| year, first molares. 10th year, second bicuspides. 



7th year, two middle incisors. llth to 12th year, canine. 

 8th year, two lateral incisors. 12th to 13th year, second molares. 

 9th year, first bicuspides. 17th to 21st year, last molares. 



* Mr. Nasrnyth is of opinion that it is "by a process of absorption, and not of disrup- 

 tion, thai the tooth is emancipated." Medico-chirurgical Transactions. 1839. 



