28 BONES, JOINTS AND LIGAMENTS 



matter is sufficient to give a maximum of strength and 

 rigidity to the bone. It therefore requires greater force 

 to break the bones of an adult and the break is com- 

 plete. In old age, the earthy material increases still 

 more in proportion to the animal matter and, as a result, 

 the bones become more brittle and are not apt to heal 

 as readily when broken. 



Shapes of bones. The forms of bones differ accord- 

 ing to their position and use in the skeleton. Those of 

 the arm, leg, palm, finger, arch of the foot, and toes 

 are called long bones, because they have relatively long 

 shafts. All the long bones have enlarged ends which 



Osteoblasts 

 New bone 



in Old bone 



FIG. 23. The formation of new bone on the surface of old bone by means of a 

 layer of osteoblasts. 



form joints. The block-shaped bones of the wrist and 

 ankle, on the contrary, have the enlarged joint ends 

 but no shafts and are therefore called short bones. The 

 vertebrae are essentially short bones but they possess so 

 many extensions or processes that they have earned 

 the name of irregular bones. The sternum and bones 

 of the skull, which are each formed of two dense bony 

 plates united by a layer of spongy (cancellated) bone, 

 are called flat or tabular bones. The ribs are also ordi- 

 narily classed as flat bones in spite of their length, be- 

 cause they are built in the same way. 



