68 GROWTH OF TEETH. 



most efficient grinder in the adult mouth, we must consider it a per- 

 manent tooth." " It is a curious circumstance, and one which will 

 readily suggest itself to the surgeon, that laying out of view the 

 wisdom teeth, which sometimes decay at an early period from 

 other causes, the anterior molars are the permanent teeth which 

 most frequently give way first, and in the most symmetrical manner 

 and at the same time, and frequently before the milk set." 



Growth of Teeth. Immediately that the dental follicles have been 

 closed by their opercula, the pulps become moulded into the form of 

 the future teeth ; and the bases of the molars divided into two or 

 three portions, representing the future fangs. The dental sac is 

 composed of two layers, an internal or vascular layer, which was 

 originally a part of the mucous surface of the mouth, and a cellulo- 

 fibrous layer, analogous to the corium of the mucous membrane. 

 Upon the formation of this sac by the closure of the follicle, the 

 mucous membrane resembles a serous membrane in being a shut sac, 

 and may be considered 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 be- 

 comes 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 secreted 

 by the pulp, and deposited upon its most prominent point : if the 

 tooth be incisor or canine, the secreted layer has the form 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 secretion of additional layers, the pulp 

 becomes gradually 'surrounded and diminishes in size, depositing 

 fresh layers during its retreat into the jaws until the entire tooth 

 with its fangs is completed, and the small cavitas pulpse of the per- 

 fect 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 secretion 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 tne 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 secretion 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 



