326 



THE DIGESTIVE SYSTEM 



dense connective tissue, the majority of whose fibers are disposed .in a 

 longitudinal direction. The meshes of the matrix are almost completely 



filled by a deposit of calcareous salts 

 which gives the dentin its bony consis- 

 tence. Dentin consists of about 28 per 

 cent, organic and 72 per cent, earthy 

 matter. The latter includes calcium 

 phosphate, about 67 per cent. ; calcium 

 carbonate, about 3 per cent. ; and mag- 

 nesium phosphate, with a trace of cal- 

 cium fluorid. 



Here and there, especially toward its 

 peripheral border and near the apex of 

 the tooth, the dentinal matrix fails to 

 become calcified. Such uncalcified areas, 

 inter globular spaces, are encroached upon 

 by the rounded or globular margins of 

 the adjacent calcified matrix which 

 forms the so-called dentinal globules. 



The dentin is everywhere permeated 

 by a system of canaliculi, the dentinal 

 tubules or canals, which extend in a ra- 

 dial manner from the pulp cavity out- 

 ward to the cementum and enamel 

 Their course is characteristically curved; 

 resembling the letter s. The cavity of 

 the dentinal tubules is partially occupied 

 by the dentinal processes (fibers) of the 

 odontoblasts, an arrangement which may 

 be compared to that existing between the 

 processes of the bone cells and the canal- 

 iculi of bone. At their inner extremity 

 the dentinal tubules are 2 to 4 p in di- 

 ameter, but they taper very gradually, 



FIG. 306. SECTION OF FANG 

 PARALLEL TO THE DENTINAL 

 TUBULES, HUMAN CANINE. 

 (Waldeyer). 



1, cementum, with large 

 lacunse, canaliculi and indica- 

 tions of lamellae; 2 t granular 

 layer of Tomes with large in- 

 terglobular spaces; 3, dentinal 

 tubules. X 300. (From Quain's 

 "Anatomy.") 



especially in the outer portion of their course, where they finally reach a 

 diameter of no more than 0.5 to 1 /*. 



The dentinal tubules may divide dichotomously in the inner third of 

 their course; beyond this point they give off very fine lateral twigs, 

 which at first leave the parent tubule nearly at right angles, but later 

 are slightly inclined outward. At their distal end most of the dentinal 



