DENTIN. 55 



The epithelial cells of the special dental germ become changed 

 into three kinds of cells : (1) Columnar cells, adamantoblasts or 

 ameloblasts. These are the deepest layer next the papilla, and 

 therefore next the future dentin. The adamantoblasts form the 

 enamel-prisms (Fig. 46), at first fibrous in character, later becoming 

 calcified. (2) The outer cells, those adjoining the dental sac, be- 

 come arranged into a single layer of cubical epithelium. Between 

 the two the cells form a spongy network of (3) branching cells, 

 whose processes communicate, forming the stellate reticulum or 

 enamel-jelly or enamel-pulp. The name enamel-organ is now 

 applied to this structure. 



The cement, which, as already stated, is identical with bone, 

 is formed by the dental sac, whose internal tissue is in all respects 

 the same as the osteogenetic layer of periosteum. The outer layer 

 of this sac is the dental periosteum. 



The above description is that of the development or formation 

 of the temporary or milk-teeth (p. 56). The permanent teeth are 

 formed in the same manner. The process from which each of 

 these latter is developed is an offshoot of the special dental germ, 

 which produces a temporary tooth, and this offshoot undergoes the 

 same changes. The milk-teeth are shed by the action of the 

 osteoclasts of the dental periosteum, here called odontoclasts, which 

 cause absorption of the roots of these teeth. 



While there are but ten temporary teeth in each jaw, there are, 

 on the other hand, sixteen permanent ones, or six more ; the perma- 

 nent molars, three on each side of the jaw, the first and secondmolars y 

 and the wisdom-teeth. These arise from a backward extension of 

 the dental germ, for which additional special germs are developed. 



The eruption or cutting of the teeth is due to the absorption of 

 the gum about them by the pressure of the growing teeth. 



The alveoli or sockets are formed by the ossification of the 

 tissue between the dental sacs. 



The ten teeth which replace the ten temporary are called succes- 

 sional permanent teeth ; the other six, superadded permanent teeth. 

 The molars of the temporary set are replaced by the pre molars 

 or bicuspids of the permanent set, while the superadded teeth are 

 the molars of the permanent set, and have no representatives in 

 the temporary set. 



While the formation of the milk-teeth begins at about the 

 seventh week of fetal life, that of the successional permanent 

 teeth commences at about the sixteenth week, the second molars 

 at the third month, and the wisdom teeth at the third year. 



Temporary, Milk-, or Deciduous Teeth. The first set of teeth, 

 ten in number in each jaw, twenty in all, constitute the temporary, 

 milk-, or deciduous teeth. Four of these are incisors, two canines, 

 and four molars. The following table gives their arrangement 

 and approximate time of eruption or cutting. 



