TEETH ERUPTION. 



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 correspond- 

 ing with the number of tubercles in 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, communicating 

 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 con- 

 version of that membrane itself into bone ; the former supposition is the 

 more probable. 



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

 viously to birth. 



Eruption. When the crow r n 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 ad- 

 hesion 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 t 1 

 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 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 w r hich 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 sur- 

 face 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 

 F;<c. 47.| 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 second- 

 ary groove, still exist, in the form of minute obliterated cords, 

 separated from the deciduous teeth by their alveolus, but com- 

 municating through a minute osseous canal with the fibrous 

 tissue of the palate, immediately behind the corresponding de- 



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

 tion, that the tooth is emancipated." Medico-chirurgical Transactions. ItvJ'J. 



f Temporary tooth with the capsule of it? permanent successor attached to it by th 

 Hubernaculum dentis. 



