SECT. I39.] THE TEETH. 297 



which cross the fibres in various directions, and which, on perpen- 

 dicular sections, appear as obliquely ascending lines or arches 

 (fig. 122); on transverse sections, as circles on the outer layers of 

 the enamel, rarely throughout the entire enamel — lines, which I 

 regard as being the expression of the lamellated mode of formation 

 of the enamel. 



The cuticle of the enamel is a calcified, structureless membrane, 

 o - ooo4'" to 00008"' thick, which, upon the surface directed towards 

 the enamel, is furnished with small depressions for the reception of 

 the extremities of the enamel-fibres, and, from its great capability 

 of resisting chemical re-agents, forms an excellent protection for 

 the crown of the tooth. 



§ 139. The cement, substantia ostoidea (fig. 119, A, c), is a cortex 

 of true osseous substance, which covers the fangs of the teeth, and 

 in many-fanged teeth, not unfrequently cements them together. It 

 commences as a very thin layer Avhere the enamel ceases, either 

 simply bordering upon, or extending for a small distance over the 

 latter, becomes thicker as it descends, and, finally, attains its 

 greatest thickness upon the ends of the fangs, and upon the alveolar 

 surfaces of the molar teeth between the fangs. In man, its inner 

 surface is very intimately connected with the dentine, without any 

 intermediate substance, so that very frequently, at least with 

 higher magnifying powers, the boundary of the two is not 

 very sharp. The outer surface is very closely surrounded by 

 the periosteum of the alveoli, less firmly by the gum, and, after 

 detachment of these soft parts, is mostly uneven, often circularly 

 striated. The cement is the least hard of the three substances of 

 the teeth, and is almost, chemically, the same as the bones. Bibra 

 found — 



In Man. In the Ox. 



Organic Substance ..... 29*42 32-24. 



Inorganic Substance .... 70-58 67-76 



IOO'OO IOO'OO 



Iii the ox, the proportions of the several ingredients were — 

 Phosphate of Lime and Fluoride of Calcium . . 58-73 



inmate of Lime 

 Phosphate of Magnesia 

 Salts 



Cartilage 

 Fat 



7'22 

 0-99 

 0-82 

 31-31 

 0-93 



IOO'OO 



