THE FRAME WORK OF THE BODY 19 



by their union we are able to withstand greater shocks than 

 would be possible with either alone. In youth, the animal 

 portion is in excess; a bone then does not break so readily, 

 but, when broken, unites with great rapidity and strength. 

 On the other hand, the bones of old persons are more easily 

 broken, and in some cases fail to unite. The mineral matter 

 being then in excess indicates that the period of active exer- 

 tion is drawing to a close. (Read Note 1.) 



5 The Structure of the Bones. If we examine one of the 

 long bones, which have been sawed through lengthwise, we 



FIG. 1. SECTION OF BONE. A, Longitudinal. 

 B, Transverse 



observe that it is admirabFy fashioned for affording lightness 

 as well as strength (Fig. 1). Its exterior is hard and resisting, 

 but it is porous at the broad extremities, while through the 

 central portion there is a cavity or canal which contains an 

 oily substance called marrow. The broad extremities furnish 

 abundant surface for the formation of joints and for the 

 attachment of muscles; they are porous so that weight may 



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." 



5. The structure of the bones ? 



