THE STRUCTURE OF THE ELEMENTARY TISSUES. 75 



cess is complete, entirely hidden by the densely deposited lime salts" 

 (Mummery). 



In their course from the pulp-cavity to the surface the minute tubes 

 form gentle and nearly parallel curves and divide and subdivide dicho- 

 tomously, but without much lessening of their calibre until they are 

 approaching their peripheral termination. 



From their sides proceed other exceedingly minute secondary canals, 

 which extend into the dentine between the tubules and anastomose with 

 each other. The tubules of the dentine, the average diameter of which 

 at their inner and larger extremity is ^Vo of an inch, contain fine pro- 

 longations from the tooth-pulp, which give the dentine a certain faint 

 sensitiveness under ordinary circumstances and, without doubt, have to 

 do also with its nutrition. These prolongations from the tooth-pulp 

 are probably processes of the dentine-cells or odontoblasts which are 

 branched cells lining the pulp-cavity; the relation of these processes to 

 the tubules in which they lie being precisely similar to that of the pro- 

 cesses of the bone-corpuscles to the canaliculi of bone. The outer portion 

 of the dentine, underlying the cement, and the enamel to a much lesser 

 degree, forms a more or less distinct layer termed the granular or in- 

 terglobular layer. It is characterized by the presence of a number of 

 irregular 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. 75, b, c), 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 

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

 ever, amounts only to about 2 or 3 per cent. It contains a larger pro- 

 portion 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. 76, 77) -goVo f an incn in diameter, 

 which are set on end on the surface of the dentine, and fit into corre- 

 sponding depressions in the same. 



They radiate in such a manner from the dentine that at the top of 

 the tooth they are more or less vertical, while toward the sides they tend 

 to the horizontal direction. Like the dentine tubules, they are not 

 straight, but disposed in wavy and parallel curves. The fibres are 

 marked by transverse lines, and are mostly solid, but some of them may 

 contain a very minute canal. 



The enamel-prisms are connected together by a very minute quantity 

 of hyaline cement-substance. In the deeper part of badly formed en- 



