220 HUMAN ANATOMY. 



Structure. On section a tooth consists of two portions, the 

 pulp cavity and the solid portion surrounding it. 



The solid portion consists of three structures, dentine, or 

 ivory, which forms the principal mass of the tooth; enamel, 

 which covers the crown, and cement, which covers the surface 

 of the fang. 



Dentine, or ivory, resembles bone, but differs from it in 

 composition and structure, consisting of twenty-eight parts ani- 

 mal and seventy-two parts mineral matter, and being made up 

 of minute tubuli held together by the intertubular substance. 



The dental tubuli are minute cylindrical canals 1 / 4r , o of an 

 inch in dfameter. They pass in a spiral direction from the pulp 

 cavity to the periphery. 



Enamel, the hardest and densest of all organized bodies, con- 

 tains but 3.5 per cent, animal matter, and is composed of minute 

 hexagonal rods Vssoo f an i nc h i n diameter, placed at right 

 angles to the surface of the dentine. The external surface of 

 unworn enamel can be separated as a thin, homogeneous mem- 

 brane, Nasmytli's membrane. 



Cement, or crusta petrosa, is a thin layer of true bone with 

 canal iculi and lacunae, disposed on the surface of the fang. 



The pulp cavity is a cavity within the base of the crown, 

 continuous with a canal in the centre of the fang, and open at 

 the apex of the fang for the entrance of vessels and nerves. It 

 is filled with dental pulp. 



Dental pulp consists of two kinds of cells, the fusiform and 

 the columnar, or odontollasts of Waldeyer, held together by 

 loose connective tissue. It is soft, vascular, and highly sensitive. 

 The nerves are both medullated and non-medullated, and form a 

 rich plexus beneath the odontoblastic layer. The terminal fibrils 

 probably unite with these cells, but the exact distribution is 

 still unsettled. 



Development Temporary. They are formed very early, 

 seven to eleven weeks, in the primitive dental groove by an in- 

 volution of the epithelium of the oral cavity covering the maxil- 

 lary arches into the blastema or corium and connective tissue 

 below, the former forming the enamel, the latter the cement 

 and dentine. 



The enamel is formed by the enamel germ (a mass of epi- 

 thelial cells) descending into the dental groove until it meets the 

 papilla, a vascular growth extending upward from the connective 

 tissue, upon which it forms a cap. A vascular membrane in- 

 closing the enamel germ then extends itself as the dentinal 

 sac upon the united papilla and enamel germ and cuts the latter 

 off from its former epithelial structure. The cells become dif- 

 ferentiated and finally calcify. 



