70 OS HYOIDES. 



space between the extremity of the unfinished root and the bottom of 

 the socket, in which the growth and completion 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 al- 

 veoli. 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 obliterated cords, separated 

 from the deciduous teeth by their alveolus, but communicating through 

 a minute osseous canal with the fibrous tissue of the palate, im- 

 mediately 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 development of animal 

 bodies, viz. that parts, or organs, ^vh^ch have once acted an important 

 part, however atrophied they may afterwards become, yet never alto- 

 gether disappear, so long as they do not interfere with oilier 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. 1 8th month, canine. 



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



1 2th month, first molares. 



The periods for the permanent teeth are, 



65 year, first molares. 10th year, second bicuspides. 



7th year, two middle incisors. llth to 1 2th year, canine. 



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



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



OS HYOIDES. 



The os hyoides forms the second arch developed from the cranium, 

 and gives support to the tongue, and attachment to numerous muscles 

 in the neck. It is named from its resemblance to the Greek letter v, 

 and consists of a central portion or body, of two larger cornua, which 



