480 SPECIAL HISTOLOGY. 



enamel, rarely extending through its whole thickness. These I regard 

 as the expression of the lamellated development of the enamel. 



Nasmyttis membrane is a calcified, structureless membrane, 0-0004- 

 0-0008 of a line thick, distinguished by the great resistance it offers to 

 chemical reagents, and its consequent appropriateness as a defence for 

 the crown of the tooth. It is not altered by maceration in water, and 

 is not dissolved by boiling in water, concentrated acetic acid, hydro- 

 chloric acid, sulphuric acid, and nitric acid ; the latter only renders it 

 yellow. In caustic alkalies it remains unchanged. Boiled with caustic 

 potassa and soda, it becomes white and somewhat disintegrated, but still 

 forms a connected mass ; the potassa is rendered slightly turbid by the 

 addition of hydrochloric acid, but clears with an excess. The membrane 

 burns with an ammoniacal odor, and yields a calcareous spongy coal. 



140. The Cement, substantia ostoidea, cementum (Fig. 185), is a 

 cortical layer of true osseous tissue, which covers the fangs, and in the 

 many-fanged teeth, not uncommonly unites them all together. It com- 

 mences where the enamel ceases, as a very thin layer, either simply 

 abutting upon, or slightly overlapping it, increases in thickness lower 

 down, and finally attains its maximum at the ends of the fangs, and on 

 the alveolar surface of the molar teeth between the fangs. Its internal 

 surface is, in man, very intimately united with the dentine, but without 

 any connecting substance, so that frequently, at least under high powers, 

 the limit of the two structures is not altogether sharply defined. The 

 external surface is very closely surrounded by the periosteum of the 

 alveolar cavity, but is not so firmly united with the gum ; after the 

 removal of these soft parts it is usually irregular, and is frequently 

 marked with annular striations. The cement is the least hard of the 

 three dental tissues, and is, chemically, almost identical with bone. 

 Von Bibra found : 



Organic matters, ..... 

 Inorganic matters, . . . 



100-00 10000 



The latter, thus composed : 



Phosphate of lime and fluoride of calcium, 58-73 



Carbonate of lime, 7'22 



Phosphate of magnesia, 0'99 



Salts, 0-82 



Cartilage, 31-31 



Fat, 0-93 



100-00 



The earthy salts are readily extracted from the cement by acids, a 



