STRUCTURE OF THE BONE. 81 



The composition of bone is chiefly phosphate of lime, 

 cemented into a solid form by animal matter. On ex- 

 posure to heat, bone becomes black, in consequence of 

 the conversion of this animal matter into charcoal. In 

 the mean time the oil contained in the cavities takes fire, 

 and all the combustible materials of which the bone is 

 composed are consumed. It now becomes white and 

 porous, having by the process lost about half its weight. 

 What remains, being as it were the skeleton of perfect 

 bone, is phosphate of lime deprived of its animal ce- 

 ment. It is now so brittle as to be broken by a light 

 blow, or even ground to powder in a mortar. On break- 

 ing the bone across, we are now able to discover the 

 cavities which contained the oily matter, and probably 

 also some of those which contained the animal cement. 



On the contrary, by steeping a bone in a quantity of 

 acid, sufficiently diluted to prevent its action on the ani- 

 mal matter, we may deprive it of its phosphate of lime, 

 and preserve this matter entire. The substance remain- 

 ing after the solid matter has been dissolved, retains the 

 exact form of the bone, but is soft, flexible, and elastic ; 

 and is resolvable into a jelly, by boiling. This substance 

 is very nourishing, and is that which forms the soup 

 made from bones. 



The different mechanical purposes for which the bones 

 of the living system are employed, require that they 

 should be of a great variety of forms. Thus the spine, 

 or principal support of all the vertebrata, requiring mo- 

 tion in all directions, is made up of a great number of 

 pieces, joined together by a layer of cartilage between 

 each two, which by its elasticity allows of the required 

 motions. The wrist and ankle are also composed of 

 many pieces each, also allowing of general motion. On 

 the contrary, the limbs acting as a system of levers, con- 

 nected by joints, are composed of solid, firm pieces, gen- 

 erally of a cylindrical form, and of considerable length, 



What is the composition of bone 1 Why does burning bone become 

 black 1 Why does burned bone become white 7 When a bone is steep- 

 ed in acid, what part of its composition is destroyed 1 What is the ap- 

 pearance, and what the consistence of the bone, when the phosphate of 

 lime is destroyed 7 What is said of the different forms of bones 7 



