GROWTH OF BONE 



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Building of bone. The structure of bone can best 

 be understood, if its growth and development are 

 studied. The long bones take shape in a gristly sub- 

 stance called cartilage, which like collagen yields gela- 

 tine when boiled. The cartilage is then stiffened by a 



FIG. 24. A small piece of bone, ground very thin and highly magnified, showing 

 central canals for blood-vessels and bone cells with connecting filaments. 



deposit of lime salts, and at the time of birth this 

 stiffened cartilage constitutes the bone. Soon after birth, 

 this is eaten into by large independent cells, 1 which 

 make a sort of honeycomb of it. Another army of cells 2 

 builds this honeycomb full of layers of bone, in which 

 many of them become buried. The original cartilage 

 which formed the wall of the honeycomb is also eventu- 

 ally changed into true bone. 



Growth of long bone in length. The way in which 

 the bones when once formed grow, depends somewhat 



1 The osteoclasts. * The osteoblasts. 



