1214 



THE ORGANS OF DIGESTION 



Chemical Composition. According to Berzelius and Bibra, dentine consists of twenty-eight 

 parts of animal and seventy-two of earthy matter. The animal matter is resolvable by boiling 

 into gelatin. The earthy matter consists of phosphate and carbonate of calcium, with a trace of 

 fluoride of calcium, phosphate of magnesia, and other salts. 



The cementum, or crusta petrosa (substantia ossea) (Fig. 924), is disposed as a thin layer 

 on the roots and neck of a tooth, from the termination of the enamel as far as the apex of 

 the root, where it is usually very thick. At the neck it overlies a slight margin of enamel. In 

 structure and chemical composition it is true bone. It consists of a number of lamellae that are 

 thicker near the apex than the neck of the tooth in young teeth. In older teeth there are addi- 

 tional shorter supplemental lamellae at the apex. Between the lamellae are the lacunas and 

 canaliculi, and it is claimed that in thick cementum even Haversian systems may be found. 

 It is composed of about 66 per cent, organic matter and 34 per cent, inorganic matter. The 

 teeth of the young usually contain Haversian systems in the thicker portions of the cementum. 

 The neck of the tooth does not contain lacunae. Sharpey's fibres (p. 38) are very numerous. 

 Some of the lacunae of the cementum receive dentinal tubes from the dentine. 



As age advances the cement increases in thickness, and gives rise to those bony growths, or 

 exostoses, so common in the teeth of the aged; the pulp cavity becomes also partially filled up 

 by a hard substance intermediate in structure between dentine and bone (the osteodentine of 

 Owen; the secondary dentine of Tomes). It is formed by the odontoblasts, the dental pulp 

 lessening in volume. 



FIG. 928. Enamel prisms (350 diameters). A. Fragments and single fibres of the enamel isolated by the action 

 of hydrochloric acid. B. Surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 



The pulp (pulpa dentis) occupies the pulp cavity or chamber. This chamber communicates 

 with the outside through a minute canal in the apex of each fang, called the root canal (canalis 

 radicis dentis). The foramen at the apex is called the apical foramen (foramen apicis dentis). 

 The pulp is a highly vascular and sensitive mass of mucous connective tissue connected with 

 the nutrition and sensitiveness of the tooth. It consists of a network of delicate fibrils sup- 

 porting spindle-shape, stellate, and spheroidal cells, and, lastly, the odontoblasts, which are 

 arranged in a single row upon the surface of the pulp; each odontoblast is a cylindrical or flask- 

 shaped cell possessing several processes, of which the peripheral becomes the dentinal fibre. 

 The bloodvessels of the pulp break up into innumerable capillary loops which lie beneath the 

 layer of odontoblasts. The nerve fibrils break up into numberless amyelinic filaments, which 

 spread out beneath the odontoblasts, and probably send terminal filaments to the extreme 

 periphery of the pulp outside the odontoblasts. The matrix cells and their processes are irregu- 

 larly arranged in the body of the pulp, but in the canal portion the fibrillaa are in the direction of 

 the axis of the root. 



The peridental membrane (pericementnm) is a vascular and sensitive fibrous tissue membrane 

 that holds the tooth in place. Upon its internal surface it forms cementum, while upon its outer 

 surface it forms the bone of the alveolar process of maxilla or mandible. It is thickest at the 

 apical and gingival regions of the tooth and thin in the middle. Upon its inner surface are 

 seen cementoblasts, and upon its outer surface are found osteoblasts. The arteries are derived 

 from the apical artery, and ultimately form a capillary plexus beneath the cemental and alveolar 

 surfaces of the membrane. The venous channels converge at the apex to empty into the 

 apical vein. 



Lymphatics are said to be absent. 



Development of the Teeth (Figs. 929 to 935). The teeth are an evolution from the 

 dermoid system, and not of the bony skeleton; they are developed from two of the blasto- 



