STRUCTURE OF THE ELEMENTARY TISSUES. 



57 



which they lie being precisely similar to that of the processes of the bone- 

 corpuscles 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 tlie 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 lacunae in the cement, and communicating with one 

 another and with the ends of the dentine-tubes (Fig. 60), and containing 

 cells like bone-corpuscles. 



II. Enamel. 



Chemical composition. The enamel, which is by far the hardest por- 

 tion of a tooth, is composed, chemically, of the same elements that enter 



a\ 1* 



FIG. 61. 



FIG. 02. 



FIG. 61. Thin section of the enamel and a part of the dentine, a, cutieular pellicle of the enamel; 

 ft, enamel fibres, or columns with fissures between them and cross striae: c, larger cavities in the 

 enamel, communicating with the extremities of some of the tubuli (d). X 350. (Kolliker.) 



FIG. 6-2. Enamel fibres. 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. X350. (Kolliker.) 



into the composition of dentine and bone. Its animal matter, however, 

 amounts only to about % or 3 per cent. It contains a larger proportion of 

 inorganic matter and is harder than any other tissue in the body. 



Structure. Examined under the microscope, enamel is found com- 

 posed of fine hexagonal fibres (Figs. 61, 62) - rT5 V?r f an i ncn * n diameter, 



