STRUCTURE Ob' THE BONE. 77 



215. The composition of bone is chiefly phosphate of 

 lime, cemented into a solid form by animal matter. On 

 exposure to heat, bone becomes black, in consequence of 

 the conversion of this animal matter into charcoal. In 

 the meantime 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 cement. 

 It is now so brittle as to be broken by a light blow, or 

 even ground to powder in a mortar. On breaking 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. 



216. 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. 



217. 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 motion 

 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, 

 having motion only at the points of connection. Lev- 



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

 black? Why does burnt bone become white ? When a bone is steep- 

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

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

 me is destroyed ? What is said of the different forms of bones f 



7* 



