DIGESTIVE SYSTEM. 155 



filling. The arrangement of the enamel rods must 

 therefore be constantly borne in mind in the opera- 

 tive repair of teeth. 



Dentin. The dentin is the second layer of teeth, 

 and not only makes up the mass of a tooth but de- 

 termines its form; that is, the number of cusps and 

 roots is moulded by the developmental process of the 

 dentin. Its histologic form has much to do with the 

 penetration of caries. 



Dentin, like bone, develops from the mesoderm, 

 and consists of an organic, formative matrix impreg- 

 nated with about 72 per cent, of inorganic salts. On 

 boiling it yields gelatin. Minute canals of dentinal 

 tubules radiate from the central cavity of the tooth, 

 which contains the formative organ or pulp. These 

 tubules are from i.i to 2.3 microns in diameter and 

 are separated from each other by a dentinal matrix of 

 about 10 microns in diameter. In the crown the 

 tubules branch but little, excepting close to the en- 

 amel where they anastomose freely. In the crown 

 they radiate in sweeping curves so as to open at 

 right angles on the dentinal surface. This produces 

 "s" or "f "-shaped curves known as primary curves. 

 They also present many wavy curves known as 

 secondary curves, which is really the result of an open 

 spiral course taken by the tubules. In the body of 

 the dentin a few small branches are given off at 

 acute angles, but near the enamel junction the tu- 

 bules fork and branch freely, forming an anasto- 

 mosis that facilitates in the spreading of caries just 

 beneath the enamel, the micro-organisms diffusing 

 sideways and then penetrating the dentin in the 

 direction of the tubes. 



