DIGESTIVE ORGANS AND THEIR ANATOMY. 273 



prisms result from a direct calcification of these columnar 

 cells themselves, and in this way they account for the hex- 

 agonal shape of the prisms. Other anatomists hold that the 

 enamel prisms are formed from secretions which these cells 

 produce at one end. It is impossible to determine just 

 which of these two views is correct, the probability being 

 that the cells themselves become hardened into the prisms, 

 just as in the case of the finger nail similar epithelium cells 

 become hardened into these horny structures. The upper 

 layers of cells of this lateral outgrowth become changed into 

 the enamel cuticle which was referred to as covering a per- 

 fectly new tooth. 



Formation of Dentine. As soon as the enamel crown 

 begins to arise in the way just indicated, cells appear im- 

 mediately underneath it, which begin to deposit dentine 

 next to the enamel. These cells, called dentinoblasts, or 

 by others odontoblasts, secrete a matrix which hardens at 

 once into the ivory. In this matrix these cells extend pro- 

 toplasmic processes which become imbedded in the dentine, 

 and which in the fully formed tooth are the protoplasmic 



Fig. 105. SECTION OF DEVELOPING DENTINE FROM AN INCISOR OF A YOUNG RAT. (After 



Schafer.) 



a, outer layer of fully formed dentine ; &, soft matrix, with a few nodules of calcareous 

 matter already in it ; c, odontoblasts with arms extending into the dentine ; d, pulp cavity 

 with contained pulp. 



processes lying in the dentinal tubules. These dentino- 

 blasts, however, do not wall themselves in as in the case of 

 bone, but always remain outside of the dentine in the pulp 

 cavity. Thus the dentine surrounding one or more den- 

 tinal tubules from the enamel entirely down to the pulp 

 18 



