DIGESTION. 



225 



which extend into the dentine between the tubules, and anastomose with 

 each other. The tubules of the dentine, the average diameter of which 

 at their inner and larger extremity is ^^ir of an inch, contain fine pro- 

 longations from the tooth-pulp, which 

 give the dentine a certain faint sensitive- 

 ness under ordinary circumstances and, 

 without doubt, have to do also with its 

 nutrition. These prolongations from the 

 tooth-pulp are really processes of the den- 

 tine-cells or odontoUasts, which are 

 branched cells lining the pulp-cavity ; the 

 relation of these processes to the tubules 

 in which they lie being precisely similar 

 to that of the processes of the bone-cor- 

 puscles to the canaliculi of bone. The 

 outer portion of the dentine, underlying 

 both the cement and enamel, forms a 

 more or less distinct layer termed the 

 granular or inter globular layer. It is 

 characterized by the presence of a number 

 of minute cell-like cavities, much more 

 closely packed than the lacunas in the 

 cement, and communicating with one an- 

 other and with the ends of the dentine- 

 tubes (Fig. 168), and containing cells like 

 bone-corpuscles. 



FIG. 167. Prernolar tooth of cat in 

 situ. Vertical section. 1. Enamel with 

 decussating and parallel striae. 2. Den- 

 tine with Schreger's lines. 3. Cement. 

 4. Periosteum of the alveolus. 5. In- 

 ferior maxillary bone, showing canal 

 for the inferior dental nerve and ves- 

 sels, which appears nearly circular 

 in transverse section. (Waldeyer.) 



II. Enamel. 



Chemical composition. The enamel, 

 which is by far the hardest portion of a 

 tooth, is composed, chemically, of the same elements that enter into the 

 composition of dentine and bone. Its animal matter, however, amounts 



FIG. 168.-Section of a portion of the dentine and cement from the middle of the root of an incisor 



15 



