THE PALATE. 717 



milk-teeth. In the incisor and canine teeth, this newly-formed lamina has the 

 form of a hollow cone ; in the molar teeth, as many separate laminae are found 

 as there are eminences upon its crown. These laminas grow at the expense of 

 the pulp-substance, increasing in breadth by a growth round their margins, and 

 in thickness by a similar formation in its substance ; the separate cones (if a 

 molar tooth) ultimately coalesce, and the crown is completely formed. The 

 pulp now becomes constricted, so as to form the cervix ; and the remaining 

 portion becomes narrow and elongated, to form the fang. The growth of den- 

 tine takes place from the surface towards the interior, until nothing but the 

 small pulp-cavity remains in the centre of the tooth, communicating by the 

 aperture left at the point of each fang with the dental vessels and nerves. 



As soon as the formation of the dentine has commenced, there is developed 

 from the inner wall of the dental sac a soft pulpy mass, the enamel organ, which 

 is ultimately united to the surface of the dental pulp, or its cap of dentine. It 

 consists of a mesh of fibres, elastic and spongy, containing within its reticu- 

 lations fluid albumen ; and at the point of junction of each fibre, a transparent 

 nucleus is visible. The surface towards the dentinal pulp is covered by a layer 

 of elongated nucleated cells, the enamel membrane. The deposition of the 

 enamel takes place on the outer surface of the cap of dentine. 



The cementum appears to be formed at a later period of life, by the periodontal 

 membrane, extending from the margin of the enamel downwards. 



Eruption. When the calcification of the different tissues of the tooth is suffi- 

 ciently advanced to enable it to bear the pressure to which it will be afterwards 

 subjected, its eruption takes place, the tooth making its way through the gum. 

 The gum is absorbed by the pressure of the crown of the tooth against it, which 

 is itself pressed up by the increasing size of the fang (Fig. 394). At the same 

 time, the septa between the dental sacs, at first fibrous in structure, ossify, and 

 constitute the alveoli; these firmly embrace the necks of the teeth, and afford 

 them a solid basis of support. 



The eruption of the temporary teeth commences at the seventh month, and 

 is complete about the end of the second year, those of the lower jaw preceding 

 the upper. 



The periods for the eruption of the temporary set are : 



7th month, central incisors. 14th to 20th month, canine. 



7th to 10th month, lateral incisors. 18th to 36th month, posterior molars. 



12th to 14th month, anterior molars. 



Calcification of the permanent teeth commences a little before birth, and pro- 

 ceeds in the following order in the upper jaw, in the lower jaw a little earlier. 

 First molar, five or six months; the central incisor a little later; lateral inci- 

 sors and canine, about the eighth or ninth month; the bicuspids at the second 

 year; second molar, five or six years; wisdom-tooth, about twelve years. 



Previous to the permanent teeth penetrating the gum, the bony partitions 

 which separate their sacs from the deciduous teeth are absorbed, the fangs of 

 the temporary teeth disappear, and the permanent teeth become placed under 

 the loose crowns of the deciduous teeth; the latter finally become detached, 

 and the permanent teeth take their place in the mouth. 



The eruption of the permanent teeth takes place at the following periods, 

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



6| years, first molars. 10th year, second bicuspid. 



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



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



9th year, first bicuspid. 17th to 21st year, wisdom-teeth. 



THE PALATE. 



The Palate forms the roof of the mouth; it consists of two portions, the hard 

 palate in front, the soft palate behind. 



