DEVELOPMENT OF THE DIGESTIVE TUBE 613 



posited, the process of deposition proceeds downward from the crown toward 

 the developing root of the tooth. However, in the root area, the cellular layers 

 of the enamel organ are compressed against the dentine, where they form the 

 epithelial sheath. The sheath eventually disintegrates and disappears. The for- 

 mation of enamel thus becomes restricted to the upper or crown part of the 

 tooth, the root portion consisting only of dentine. As the root area of the 

 tooth lengthens downward, the tooth as a whole moves upward. Finally, the 

 crown of the tooth erupts to the outside through the tissues of the gum (fig. 

 286D). The eruption, completion, and shedding of the milk or deciduous 

 teeth in the human body occur apparently as shown in the following table. 



The Milk Dentition 



Median incisors 6th to 8th month 



Lateral incisors 8th to 12th month 



First molars 12th to 16th month 



Canines 17th to 20th month 



Second molars 20th to 24th month 



The Permanent Dentition 



First molars 7th year 



Median incisors 8th year 



Lateral incisors 9th year 



First premolars 10th year 



Second premolars 11th year 



Canines 13th to 14th year 



Second molars 13th to 14th year 



Third molars 17th to 40th year 



This table is taken from McMurrich, J. Playfair. 1922. Keibel and Mall, Manual of Human 

 Embryology, page 354, Lippincott, Philadelphia. 



At about the time of eruption, the tooth becomes cemented into the alveolus 

 or socket of the jaw in the following manner: 



( 1 ) The inner layer of the dental sac (fig. 286D) forms a layer of cemento- 

 blasts which deposit a coating of cementum over the dentine of the 

 root (fig. 286E). This occurs only after the epithelial sheath (enamel- 

 layer cells around the root) has been withdrawn or otherwise has 

 disappeared. 



(2) The cells of the outer layer of the dental sac become active in forming 

 spongy bone. 



(3) As the tooth reaches maturity, the two bony surfaces, i.e., the cementum 

 of the root and the spongy bone of the jaw socket, gradually begin to 

 approach each other. Then, as more cementum is deposited and more 

 spongy bone is formed, the space between the cementum and the 

 spongy bone of the alveolus becomes extremely narrow (fig. 286E). 



