TEETH. ERUPTION. 69 



a part of the mucous surface of the mouth, and a cellule-fibrous layer, 

 analogous to the corium of the mucous membrane. Upon the forma- 

 tion of this sac by the closure of the follicle, the mucous membrane 

 resembles a serous membrane in being a shut sac, and may be con- 

 sidered as consisting of a tunica propria, which invests the pulp ; and 

 a tunica reflexa, which is adherent by its outer surface with the 

 structures in the jaw, and by the inner surface is free, being separated 

 from the pulp by an intervening cavity. As soon as the moulding of 

 the pulp has commenced, this cavity increases and becomes filled 

 with a gelatinous granular substance, the enamel organ, which is 

 adherent to the whole internal surface of the tunica reflexa, but not 

 to the tunica propria and pulp. At the same period, viz. during the 

 fourth or fifth month, a thin lamina of ivory is formed by the pulp, 

 and occupies its most prominent point : if the tooth be incisor or 

 canine, the newly formed layer has the figure of a small hollow cone; 

 if molar, there will be four or five small cones corresponding with 

 the number of tubercles on its crown. These cones are united by 

 the formation, of additional layers, the pulp becomes gradually sur- 

 rounded and diminishes in size, evolving fresh layers during its 

 retreat into the jaws until the entire tooth with its fangs is completed, 

 and the small cavitas pulpae of the perfect tooth alone remains, com- 

 municating through the opening in the apex of each fang with the 

 dental vessels and nerves. The number of roots appears to depend 

 upon the number of nervous filaments sent to each pulp. When the 

 formation of the ivory has commenced, the enamel organ becomes 

 transformed into a laminated tissue, corresponding with the direction 

 of the fibres of the enamel, and the crystalline substance of the enamel 

 is secreted into its meshes by the vascular lining of the sac. 



The cementum appears to be formed at a later period of life, either 

 by a deposition of osseous substance by that portion of the dental sac, 

 which continues to enclose the fang, and acts as its periosteum, or by 

 the conversion of that membrane itself into bone ; the former supposi- 

 tion is the more probable. 



The formation of ivory commences in the first permanent molar pre- 

 viously to birth. 



Eruption. When the crown of the tooth has been formed and 

 coated with enamel, and the fang has grown to the bottom of its socket 

 by the progressive lengthening of the pulp, the formation 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 condition, 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 contraction, leaving a 



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

 of disruption", that the tooth is emancipated." Medico -chirurgical Transac- 

 tions. 1839- 



