444 PHYSIOLOGICAL CHEMISTRY. 



Frequently human bones contain calcium fluoride, which substance, 

 to the amount of 1 to 2 per cent,, is a normal constituent of the bones 

 of many animals. The organic matter of bone is called ossein, and 

 is a mixture of collagen, elastin, and an albuminoid existing in the 

 bone-cells. Collagen is a nitrogenous substance, insoluble in water, 

 but forming when treated with it under the influence of heat and 

 pressure, gelatin, an amorphous, tasteless, translucent substance, which 

 swells up in boiling water, forming on cooling a soft jelly ; an impure 

 form of gelatin is common glue. 



Teeth consist of three distinct tissues, viz., dentine, forming the 

 chief mass, in its interior being the pulp cavity; enamel, investing 

 the crown and extending some distance down the neck ; and cement, 

 covering the fangs. The composition of cement is almost the same as 

 that of bone, its organic and inorganic constituents having the rela- 

 tive proportions of 30 : 70. 



Dentine contains less water than bone and is also poorer in organic 

 matter. The following table gives the composition of the dentine of 

 an adult woman and man respectively : 



Woman. Man. 



Organic matter ossein and vessels . . . 27.61 20.42 



Calcium phosphate ...... 66.72 67.54 



Calcium carbonate 3.36 7.97 



Magnesium phosphate 1.08 2.49 



Soluble salts, chiefly sodium chloride . . 0.83 1.00 



Fat 0.40 0.58 



Enamel is distinguished by the very small proportion of water and 

 organic matter contained in it. Its average composition may be thus 

 stated: 



Water and organic matter 3.6 



Calcium phosphate and traces of fluoride .... 86.9 



Magnesium phosphate 1.5 



Calcium carbonate ........ 8.0 



Tartar is the name given to the substance which deposits from 

 alkaline saliva on the teeth. It is of a grayish, yellowish, or brown- 

 ish color, and consists chiefly of calcium phosphate, with a little 

 carbonate, but contains also some organic matter, salts of the alkalies, 

 and silica. 



Hair, nails, horns, hoofs, feathers, epithelium are nearly iden- 

 tical in composition. They all contain a nitrogenous substance, 

 termed keratin, which is probably not a distinct chemical compound, 



