492 SPECIAL PHYSIOLOGY. 



minute processes of the tooth pulp, which serve the purposes of nutri- 

 tion, and perhaps also impart sensibility to the dentine. The sub- 

 stance of the walls of the tubuli, is comparatively thick ; its structure 

 is not exactly known. The intermediate hard, or so-called intertubular 



Fig. 85. Section of a portion of the crown of a tooth, magnified about 300 diameters, d, the enamel, com- 

 posed of wavy fibres, marked with faint cross lines ; the surface is bounded with a fine homogeneous layer. 

 Beneath the. enamel, is a portion of the tooth substance, showing the ends of the tubercle of the dentine, 

 and certain irregular spaces in it. (After Kolliker.) 



substance, is slightly granular, and contains the greater part of the 

 earthy matter. When this is removed by an acid, the softened animal 

 basis is said, by some, to consist of fibres running parallel with the 

 tubes, by others, of minute corpuscles, arranged around the tubes, and, 

 according to another view, of fine lamellae disposed concentrically 

 around the pulp cavity, across the direction of the tubules, which are 

 supposed to perforate the lamellae. 



The enamel, the hardest of the dental substances, and, indeed, of 

 all known animal textures, is the dense white covering, which protects 

 the crowns of the teeth ; it is thickest on the edges of the incisor and 

 canine, and on the crown of the molar teeth, and gradually becomes 

 thinner towards the neck, where it terminates. It contains more 

 earthy matter than any other animal tissue, viz., 96.5 per cent., of 

 which 89.8 are phosphate of lime, with traces of fluoride of calcium, 

 4.4 carbonate of lime, and 1.3 phosphate of magnesia arid other salts. 

 The animal matter amounts to 3.5 per cent., the analysis showing a 

 loss of 1 per cent. (Bibra.) Berzelius estimated the animal matter at 

 the remarkably low proportion of 2 per cent. 



The enamel, Fig. 85, d, is composed entirely of microscopic hexago- 

 nal prismatic fibres, or rods, arranged closely together upon the den- 

 tine; they are fixed, by one extremity, to minute depressions on the 

 surface of the dentine, and following a somewhat wavy course, present, 

 at their outer ends, the appearance of a hexagonal mosaic pattern, 

 where they form the free surface of the enamel. On the crowns of 

 the teeth, the enamel fibres are vertical; on the sides, they become 

 first oblique, and then horizontal. Their diameter is ^g^th of an 

 inch. Near the surface of the dentine, minute interstices are found 

 between the enamel fibres, supposed to be for the purpose of nutritive 

 permeation. In the growing tooth, by the action of an acid, the ena- 

 mel may be separated into its microscopic elements, viz., into delicate 



