174 A MANUAL OF ANATOMY 



collection of mesodermic cells, which become transformed into connective 

 tissue, this being permeated by bloodvessels and nerves. 



The cells at the surface of the papilla become enlarged, and are called 

 odontoblasts. These cells are identical with the osteoblasts of developing 

 bone, and they form a continuous layer, known as the membrana eborls. 

 From these cells the dentine is laid down. The central portion of the dental 

 papilla, consisting of connective tissue, bloodvessels, and nerves, constitutes 

 the dental pulp. 



Formation of Dentine. — The dentine is laid down in successive layers. 

 Each layer consists at first of a fibrous dentinal matrix, called prodentine, the 

 fibres of which are continuous with those of the dental pulp. Subsequently 

 the matrix undergoes calcification by the deposits of calcareous matter. 

 D uring the formation of these layers the odontoblasts become shifted inwards 

 toT.-ards the centre, and they leave behind them fine processes, which become 

 invested by the calcified fibrous matrix of the dentine. In this manner the 

 dentinal tubules are formed, with processes of the odontoblasts within them, 

 which represent the fibres of Tomes. The dentine so formed constructs a 

 case for each dental papilla. The central part of the papilla, consisting of 

 connective tissue, bloodvessels, and nerves, forms, as stated, the dental pulp. 

 The peripheral part of the dentine has several small uncalcified areas, which 

 lie between globular masses of calcareous matter. These areas represent the 

 interglobular spaces of the adult tooth. 



Dental Sac. — This sac, like the dental papilla, is of mesodermic origin. 

 As the dental papilla is undergoing differentiation into dentine and dental 

 pulp, the mesodermic connective tissue around the papilla becomes condensed 

 into a follicle, called the dental sac, which encloses both the dental papilla 

 and the corresponding enamel organ. The developing tooth (enamel organ 

 and dental papilla) thus becomes isolated, the enamel organ having previously 

 lost its connection with the ectoderm of the gum by rupture of its neck. The 

 part of the dental sac which is related to the crown of the tooth is connected 

 with the surface of the gum by a band of connective tissue, called the gubcr- 

 naculum dentis. This part of the sac, along with the gubemaculum, dis- 

 appears. The part of it which is related to the root of the tooth undergoes 

 important developments. It consists of two layers — inner and outer. The 

 inner layer gives rise to the cement, or crusta petrosa, whilst the outer layer, 

 remaining fibrous, forms the alveolo-dental periosteum, or peridental membrane. 



Permanent Teeth. — The process of development of the permanent teeth is 

 similar to that of the temporary teeth. The enamel organs of those perma- 

 nent teeth [teeth of succession) which are to replace the temporary teeth — 

 namely, the permanent incisors, canines, and bicuspids — are developed as 

 thickenings or protuberances of the true dental laminae, as in the case of the 

 temporary teeth, and they grow inwards on the lingual aspects of the tem- 

 porary enamel organs. 



The enamel organs of the three permanent molar teeth spring from a back- 

 ward prolongation of the true dental lamina. 



Teeth Present at Birth. — At the period of birth the gums contain forty-four 

 teeth. These consist of — (i) the twenty temporary teeth; (2) the twenty 

 permanent teeth, which are to replace the temporary teeth; and (3) the four 

 first permanent molar teeth. At birth the second and third permanent molar 

 teeth, eight in all, are not yet developed. 



Eruption of the Temporary Teeth. — As the permanent teeth grow, they 

 exercise pressure upon the fangs of those temporary teeth which they are 

 about to replace. This continued pressure leads to partial absorption of the 

 fangs of the temporary teeth. These teeth, therefore, become loosened in 

 their alveoli, and as the permanent teeth continue to advance, the loosened 

 temporary teeth are pushed out by the advancing permanent teeth. Osteo- 

 clasts also take part in the absorption of the fangs. 



