DEVELOPMENT OF THE TEETH 1 57 



the root. The enamel cells about the future root of the tooth remain cuboidal 

 or low columnar in form^ come into contact with the outer enamel cells, and the 

 two layers constitute the epithelial sheath of the root which does not produce 

 enamel prisms (Fig. 165). 



The Dental Papilla. — The outermost cells of the dental papilla at the end 

 of the fourth month arrange themselves as a definite layer of columnar epithelium. 

 Since they produce the dentine, or dental bone, these cells are known as odonto- 

 blasts (Fig. 165). When the dentine layer is developed, the odontoblast cells 

 remain internal to it, but branched processes from them (the dentinal fibers of 

 Tomes) extend into the dentine and form the dental canaliculi. Internal to the 

 odontoblast layer, the mesenchymal cells differentiate into the dental pulp, pop- 

 ularly known as the "nerve" of the tooth. This is composed of a framework of 

 reticular tissue in wliich are found blood vessels, lymphatics, and nerve fibers. 

 The odontoblast layer persists throughout life and continues to secrete dentine, 

 so that eventually the root canal may be obliterated. 



Dental Sac. — The mesenchymal tissue surrounding the anlage of the tooth 

 gives rise to a dense outer layer and a more open inner layer of fibrous connective 

 tissue. These layers form the dental sac (Fig. 165). Over the root of the tooth 

 a layer of osteoblasts or bone forming cells develops, and, the epithelial sheath 

 formed by the enamel layers having disintegrated, these osteoblasts deposit about 

 the dentine a layer of bone which is known as the substantia ossea or cement. 

 The cement layer contains typical bone cells but no Haversian canals. As the 

 teeth grow and fill the alveoli, the dental sac becomes a thin vascular layer, con- 

 tinuous externally with the alveolar periosteum, internally with the periosteum of 

 the cement layer of the tooth. 



When the crown of the tooth is fully developed the enamel organ disinte- 

 grates, and, as the roots of the teeth continue to grow, their crowns approach the 

 surface and break through the gums. The periods of eruption of the various 

 milk or decidual teeth vary with race, climate, and nutritive conditions. Usually 

 the teeth are cut in the following sequence : 



Decidual or Milk Teeth 



Median Incisors sixth to eighth month. 



Lateral Incisors eighth to twelfth month. 



First Molars twelfth to sixteenth month. 



Canines seventeenth to twentieth month. 



Second Molars twentieth to thirty-sixth month. 



The permanent teeth are all present at the fifth year. They are located 

 mesial to the decidual teeth (Fig. 166), and, before the permanent teeth begin 



