114 



THE ALIMENTARY SYSTEM. 



which 



substance, almost entirely free from organic matter, win 

 invests the crown. It is disposed in slender stra.ght or 

 slightly wavy rods or prisms, of about six sides each, which 

 are in general perpendicular to the surface of the dentine, and 



. Jo, rr<i _ ._J, ,l,:UU nn ^T-ancvprsP 



are held together by cement. 



FIG. 47. 



The rods exhibit fine transverse 

 markings. At eruption and 

 until worn off by attrition the 

 enamel is covered with a thin 

 epithelial cuticle, the mem- 

 brane of Nasmyth, the remains 

 of the enamel-organ. 



The crusta petrosa is a thin 

 layer of bone formed over the 

 surface of the fang of the 

 tooth by the osteogenetic action 

 of the periodontal membrane 

 which invests it. It is made 

 up of bony lamellae of the 

 peripheric type, with the char- 

 acteristic lacunae and canal i- 

 culi. 



The dentine, or ivory, which 

 forms the bulk of the tooth, is 

 a connective-tissue structure, 

 impregnated and hardened by 

 calcareous salts. It contains 

 large numbers of minute chan- 

 nels or passages, the dentinal 

 tubules, straight or slightly 

 wavy, with short lateral 

 branches ; they are parallel to 

 one another, run perpendicular 

 to the surface of the dentine, 

 and open into the pulp-cavity. 

 Each tubule contains a dentinal 

 fibre, a process from one of the 

 odontoblasts or connective- 

 tissue cells in the pulp- cavity. 



Surrounding each tubule is a dentinal sheath, a membranous 

 portion of the calcified matrix especially resistant to the action 



Section of portion of tooth ^Valdeyer). 

 1, crusta petrosa, showing bony 

 lacunae and lamellae ; 2, interglobu- 

 lar spaces granular layer of Pur- 

 kinje; 3, dentinal tubules. 



