DEVELOPMENT OF THE TEETH. 115 



of acid. The curves or undulations of the various tubules 

 correspond to one another, and appear macroscopically as con- 

 centric lines, the lines of Schreger. 



The peripheral portion of the dentine contains numerous 

 stellate spaces communicating with the tubules, and in the 

 fang with the canaliculi of the crusta petrosa; these are 

 called interglobular spaces or dent'mal lacunce, and from its 

 granular appearance the zone where they occur is called the 

 granular layer of Purkinje. These lacunae are spaces left by 

 incomplete calcification, and contain (at least at an early 

 period) stellate connective-tissue cells. 



The incremental lines of Salter are, roughly, concentric lines 

 formed by interglobular spaces filled with an imperfectly 

 calcified material ; they indicate successive zones of calcifica- 

 tion of the dentine. 



The pulp-cavity is a space in the interior of the tooth en- 

 tirely enclosed by the dentine, except at the apices of the 

 fangs, where the vessels and nerves enter from the dental 

 canals in the maxillary bones. The pulp consists of delicate 

 connective tissue intermingled with bloodvessels and nerves. 

 About the periphery of the pulp-cavity, lining the dentine 

 which forms its wall, is a layer of modified connective-tissue 

 cells, called odontoblasts, long columnar or pyramidal cells 

 lying perpendicular to the dentine and sending processes, the 

 dentinal fibres, into the dentinal tubules. 



Development of the teeth: The enamel is developed from 

 epithelium, the remainder of the tooth from the subepithelial 

 connective tissue. Morphologically teeth are usually regarded 

 as modifications of the papillae of the mucous membrane. At 

 the beginning of the development of the teeth, the epithelium 

 of the mucous membrane on the edge of the jaw grows down- 

 ward into the submucous tissue, dividing into a process for 

 each tooth. These processes expand below and narrow above, 

 forming hemispherical epithelial masses, called enamel-organs, 

 whose connection with the surface-epithelium finally becomes 

 entirely cut off or reduced to a mere thread of epithelial cells. 

 Meantime beneath each enamel-organ the connective tissue 

 becomes vascular and rich in cells, forming a papilla, which 

 grows upward, pushing its way into and invaginating the 

 lower surface of the enamel-organ, so that the latter forms a 



