THE FRAMEWORK OF THE BODY 



21 



central portion there is a cavity or canal which contains an 

 oily substance, called marrow. If a thin section of bone be 



SteeSBB^feiyt ;zi.~ 



FIG. 3. STRUCTURE OF BONE 

 ENLARGED 



examined under the micro- 

 scope, we discover that it is 

 pierced by numerous fine 

 tubes (Fig. 3), about which 

 layers of bone-substance 

 are arranged. So that, al- 

 though a bone be as hard as 

 stone externally, it is by no 

 means as heavy, by reason 



nf ifo licrhf informr fpvhivP FlG - 4. SECTION PARALLEL TO THE SURFACE FROM 

 re> THE SHAFT OF THE FEMUR. -MAGNIFIED 100 

 Another element Of power DIAMETERS. a, Haversian canals ; &, c, lacunae 



is found in the curved out- 

 line of the bones. The curved line is said to be " the line of 

 beauty," as it certainly is the line of strength, and is uniformly 



and under different social conditions. In the disease called 'rickets,' 

 quite common among the ill-fed children of the poor in Europe, but some- 

 what rare in America, there is an inadequate deposit of the mineral 

 substance, rendering the bones so flexible that they may be bent almost 

 like wax. In females and weak men the bones are light and thin, while 

 in a powerful frame they are dense and heavy. Exercise is as necessary to 

 the strength of bone as to the strength of muscle ; if a limb be disused, 

 from paralysis or long sickness, the bones lose in weight and strength as 

 well as the soft parts. Bone is said to be twice as strong as oak, and, to 

 crush a cubic inch of it, a pressure equal to -5000 pounds is requisite." 



-*< 



