148 GROWTH OF BONES. 



however, according to the period of life. In in^ 

 fancy, the animal portion greatly predominates, and 

 consequently the bones are at that age compara- 

 tively soft, yielding, and elastic. In middle life, the 

 proportions are more equally balanced, and while 

 the bones thereby acquire great hardness and solid- 

 ity, they still preserve some elasticity. In old age, 

 on the contrary, when the earthy constituents pre- 

 dominate, they become dry, brittle, and compara- 

 tively lifeless. 



If a bone be subjected for a time to the action of 

 muriatic acid, the earthy portion is gradually de- 

 composed, and a cartilaginous-looking substance of 

 the exact shape and size of the bone is procured, 

 which is in reality its animal constituent. If, on the 

 other hand, the bone be subjected to the action of 

 fire, which decomposes and dissipates the animal 

 elements but scarcely affects the earths, a white, 

 light, easily crumbled mass, of the exact shape and 

 appearance of the original bone, is procured, which 

 is simply the earthy part of bone, deprived of its 

 connecting membrane. The latter is called the an 

 imal constituent of the bone, because it is the pro- 

 duct of animal life, and does not exist in nature, ex- 

 cept in the system of animals ; and the former is 

 called the earthy constituent, because it may and 

 does exist in nature, without relation to life. 



A very important purpose is served by the differ- 

 ent proportions which the animal elements of bone 

 bear to the earthy, at different ages. In early youth, 

 when much strength is not wanted, as the body is 

 never exposed to severe efforts, but when a great 

 growth of bone is required to complete the develop- 

 ment of the human frame, the animal or living part 

 of the bone is observed to preponderate. But in 

 middle life, when growth is finished, and the powers 

 of resistance are at their maximum, and when nu- 

 trition is required only to repair waste, a larger pro- 

 portion of the solid or earthy, and a smaller proper 



