22 THE FRAMEWORK OF THE BODY. 



12. The abundant supply of intervertebral cartilage has 

 another important use, namely, it adds greatly to the 

 elasticity of the frame. It is due, in part, 

 to this elastic material, and in part to the 

 frequent curves of the spine, that the brain 

 and other delicate organs are protected from 

 the shock of sudden falls or jars. During 

 the day, the constant pressure upon these 

 joints, while the body is erect, diminishes 

 the thickness of the cartilages; so that a 

 person is not so tall in the evening as in 

 the morning. The effects of this compres- 

 sion pass away when the body reclines in a 

 horizontal position. 



13. The Growth of Bone. Bone, like 

 all the other tissues of the body, is con- 

 stantly undergoing change, old material 

 being withdrawn, and new particles taking 

 their place. This has been shown conclu- 

 sively by experiments. If an animal be fed 

 with madder a red coloring matter for a 

 day or two, the bones soon become tinged ; 

 then, if the madder be discontinued for a 

 few days, the original color returns. If, how- 

 ever, this material be alternately given and 

 withheld, at short intervals, the bone will 

 be marked by a succession of red and white 

 rings. In very young animals, all the bones 

 become colored in a single day; in older 

 FIG. 6. THE ones, a longer time is required. The process 

 of waste and repair, therefore, is constantly 

 taking place in this hard substance, and with astonishing 

 rapidity. 



1 2. Elasticity of the frame ? Protection of the brain from shocks Tallness 

 of persons ? Effects of reclining ? 



13. Change in bone? Example animal and madder. Rapidity of change in 

 color? Waste and repair? 



