880 



THE ORGANS OF DIGESTION. 



globules, and irregular masses of it extruded as cementing substance. The deposi- 

 tion continues until the enamel-cap has its typical form. The deposition of the 

 layers of globules is indicated by parallel lines transverse to the axes of the enamel- 



coat 



papillae ; C, layer of ameloblasts containing enamel globules and droplets of calcoglobulin ; 1), enamel-globules 

 deposited ; E, formed dentine ; F, forming dentine ; G, layer of odontoblasts ; H, blood supply to odonto- 

 blastic layer. 



rods. At the completion of amelification the ameloblasts are partially calcified and 

 form the cuticula dentis or Nasmyth's membrane. 



Formation of Dentine. The layer of columnar cells bounding the periphery of 

 the pulp (the odontoblasts) are in apposition with 

 a plexus of capillary vessels (Fig. 480, A). Each 

 cell is a secreting body which selects the mate- 

 rial for dentine-building. Against the layer of 

 ameloblasts covering the dental papilla the odon- 

 toblasts deposit globules of the calcium albu- 

 minate, and, receding as the deposits are made, 

 leave one or more protoplasmic processes in 

 the calcic deposit (Tomes's fibres). The process 

 continues until the normal dentine thickness is 

 formed. The deposit is laid down in a scaffold- 

 ing of finely fibrillated tissue. The layer of 

 formative cells remains constant. 



Formation of Cementum. Hertwig asserts 

 that the epithelial edge of the enamel-organ 

 formed by the inner and outer epithelial layers 

 of the organ grows downward, or rather the 

 developing tooth grows upward until the future root-form of the tooth is outlined 

 by a double layer of epithelial cells (the root-sheath of Hertwig). The growth of 

 alveolar process is synchronous. 



Upon the pulp side of the sheath a layer of odontoblasts is developed ; upon 

 the outer side the fibrous encasement becomes closely attached to the sheath and a 

 layer of osteogenetic cells (cementoblasts) is differentiated. The growth of the 

 dentine of the root is the same as in the crown. The epithelial sheath undergoes 

 atrophic changes, leaving epithelial whorls which remain in the pericementum. 

 The cementum is developed as subperiosteal bone. The cementum over the apex 

 of the root is not formed until after the eruption of the tooth. 



Formation of Alveoli. By the time the crowns of the teeth have formed, each 



FIG. 481. Part of section of developing 

 tooth of young rat, showing the mode of 

 deposition of the dentine (highly magni- 

 fied), a Outer layer of fully-calcified den- 

 tine, b. Uncalcified matrix with a few nod- 

 ules of calcareous matter, c. Odontoblasts 

 with processes extending into the dentine. 

 d. Pulp. The section is stained with car- 

 mine, which colors the uncalcified matrix, 

 but not the calcified part. 



