54 SPECIAL HISTOLOGY. 



Nasmyth's membrane is a calcified, structureless membrane, 

 00004 — O'OOOS'" thick, distinguished by the great resistance 

 it offers to chemical re-agents, and its consequent appropriate- 

 ness 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, hydrochloric acid, sulphuric acid, and 

 nitric acid ; the latter only renders it yellow. In caustic alkalies 

 it remains unchanged. Boiled with caustic potass and soda, 

 it becomes white and somewhat disintegrated, but still forms 

 a connected mass ; the potass is rendered slightly turbid by 

 the addition of hydrochloric acid, but clears with an excess. 

 The membrane burns with an ammoniacal odour, 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 commences 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 sur- 

 rounded 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 : 



In Man. In the Ox. 



Organic matters 29-42 3224 



Inorganic matters 70-58 67*76 



10000 10000 



The latter, thus composed • 



