30 BONES, JOINTS AND LIGAMENTS 



upon whether they are long, short, or flat. The increase 

 of the long bone in length is provided for by a continu- 

 ous growth of cartilage just within the ends, or epiph* 

 yses, of the bone, which as fast as it grows is honey- 

 combed and built into true bone. At the age of from 

 seventeen to twenty, the cartilage ceases to form and 

 bone growth stops. 



Growth in thickness. The growth in diameter of 

 the bone from the slenderness of childhood to the thick- 

 ness of the adult, takes place under the tough mem- 

 brane, the periosteum, which encloses the entire bone. 

 The periosteum deposits layers of bone beneath itself, 

 thus building up the outside as it grows in length. As 

 the bone becomes thicker through the deposits by the 

 periosteum, its central portion is hollowed out. The 

 central or medullary canal running through the shaft of 

 each long bone is thus formed and kept proportionate to 

 its size. In this way, the size and strength of the bone 

 are greatly increased, whereas its weight is kept down. 1 



Growth of short bone. The growth of short bones 

 is in general similar to that of long. Their structure, 

 however, is spongy rather than dense and they have no 

 definite central canal. 



Growth of flat bone. The flat bones develop chiefly 

 from the periosteum, instead of from the cartilage which 

 forms the starting point of both long and short bones. 



Ends of bones. The enlarged ends of bones which 

 form joints have a spongy structure with no central 

 canal. When these ends are sawed open, it is seen that 

 the sponginess is due to an arrangement of interlacing 



1 In most animals, the central canal is filled, if the animals are 

 young, with blood-forming tissue, the red marrow; if the animals 

 are mature, with fat, the yellow marrow. In birds, the bones are 

 partially filled with air, thus giving greater lightness. 



