336 



THE DIGESTIVE SYSTEM 



d.c. 



tissue cells of embryonal or gelatinous connective tissue. They are, how- 

 ever, inclosed on all sides by the epithelial cells of the inner and outer 

 enamel epithelium and, like other epithelial tissues, are never penetrated 

 by blood-vessels. 



The enamel pulp appears to serve a purely mechanical function, it 

 being a soft tissue through which the grow- 

 ing tooth readily pushes its way to the sur- 

 face. 



The Dental Papilla. The dental pa- 

 pilla is a mesenchymal structure which is in- 

 vested by and grows into the enamel organ. 

 Coincident with the appearance of the amelo- 

 ' p ' blasts in the enamel organ, the superficial 

 cells of the dental papilla become enlarged, 

 elongated, and so arranged as to form a con- 

 tinuous layer of odontoblasts on the surface 

 of the papilla. These cells apparently secrete 

 a thin homogeneous layer, membrana pre- 

 formativa (Raschkow), which serves as a 

 basement membrane upon which the amelo- 

 blasts deposit the enamel prisms; it also 

 forms the anlage of the granular layer of 

 Tomes. 



The odontoblasts now form the DENTIL 

 in a manner entirely analogous to the de- 

 posit of bone by the osteoblasts, processes of 

 the odontoblasts being included within the 

 deposit of dentin to form the dentinal fibers. 



Irregular spaces, occurring in the dentin and granular layer, in which 

 no calcification occurs produce the inter globular spaces. 



The central mass of the dental papilla develops the reticular tissue 

 of the dental pulp. The blood-vessels and nerves enter the pulp through 

 the base of the papilla, which thus becomes the anlage of the root canal. 

 The CEMENTUM is formed by intramembranous ossification occurring 

 in that portion of the dental sac which invests the base of the dental 

 papilla and the primitive root of the tooth. 



The process of cementogenesis is essentially similar to that of peri- 

 chondrial ossification. The vascular mesenchyma of the dental sac 

 breaks through the epithelial sheath (the root extension of the enamel 

 organ), and through the agency of modified mesenchymal cells similar 



FIG. 317. ODONTOBLASTS 

 AND DENTIN OF THE 

 TOOTH OF A NEW-BORN 

 CAT. 



o, odontoblasts; Tp, Tomes' 

 fibrils, prolongations of the 

 odontoblasts; dc, dentinal 

 canal in the dentin (d). 

 X 500. (After Prenant, 

 Bouin and Maillard.) 



