THE BONE. 



1101 



Chemical Composition. Bone consists of an animal and an earthy part inti- 

 mately combined together. 



The animal part may be obtained by immersing the bone for a considerable 

 time in dilute mineral acid, after which process the bone comes out exactly the 

 same shape as before, but perfectly flexible, so that a long bone (one of the ribs, for 

 example) can easily be tied in a knot. If now a transverse section is made (Fig. 

 630), the same general arrangement of the Haversian canals, lamellae, lacunae, and 

 canaliculi is seen, though jiot so plainly, as in the ordinary section. 



The earthy part may be obtained separate by calcination, by which the animal 

 matter is completely burned out. The bone will still retain its original form, but 



Perforating fibres of 

 """ Sharpey. 



Apertures from 

 ^3 which the fibres hare 

 been withdrawn. 



Decussating fibres 

 in lamella. 



FIG. 629. Lamellae torn from a "decalcified human parietal bone to show the perforating fibres of Sharpey. 

 (Copied from a drawing by Allen Thomson.) 



it will be white and brittle, will have lost about one-third of its original weight, 

 and will crumble down with the slightest force. The earthy matter confers on 

 bone its hardness and rigidity, and the animal matter its tenacity. 



The animal basis is largely composed of ossein, which is identical with the 

 collagen of white fibrous tissue, so that when boiled with water, especially under 

 pressure, it is almost entirely resolved into gelatin. 



The organic matter of bone forms about one-third, or 33.3 per cent.; the 

 inorganic matter, two-thirds, or 66.7 per cent. Of the earthy matter, five-sixths 

 is calcium phosphate, the remainder consisting of calcium carbonate, calcium 

 fluoride, calcium chloride, and magnesium 

 phosphate, with small amounts of sodium 

 chloride and sulphate. Even after the 

 removal of all the marrow a small percent- 

 age of fat is still found in bone. 



Some of the diseases to which bones are 

 liable mainly depend on the disproportion 

 between the two constituents of bone. 

 Thus in the disease called rickets, so com- 

 mon in the children of the poor, the bones 

 become bent and curved, either from the 



Superincumbent weight Of the body Or the earthy matter by the action of acids. 



under the action of certain muscles. This 

 depends upon some defect of nutrition by which bone becomes deprived of its 

 normal proportion of earthy matter, while the animal matter is of unhealthy 

 quality. In the vertebrae of a rickety subject Bostock found in 100 parts 79.75 

 animal and 20.25 earthy matter. 



Development of Bone. In the foetal skeleton some bones are preceded by 



FIG. 630. Section of bone after the removal of 



