MOTION AND LOCOMOTION 85 



tear it on the convex side and crush it on the concave 

 side, and also to tear the roots out of the soil with 

 a rectilinear strain. The long bones of the leg, 

 owing to their being jointed together, are not sub- 

 jected to extreme tearing forces, but have to with- 

 stand more crushing, since they support the weight 

 of the body. Broad expansions, such as the leaves 

 of a plant, have to resist rupture at their edges ; 

 the body of the animal, on the other hand, being 

 much more compact than that of the plant, is not 

 subject to such stresses. A simple engineering 

 example will make 

 the arrangement of 

 skeletal material in 

 the two cases ob- 

 vious at once. 



Suppose that we 

 rest a bar of wood 

 or other elastic ma- 

 terial on two sup- FIG. 37. Principle of the girder, 

 ports as represented 



in Fig. 37, and place a heavy weight in the centre. 

 If the weight be sufficiently heavy, the bar will be 

 bent so that the under side becomes convex and the Principle 

 upper side concave. Careful measurement reveals 

 the fact that the underside has increased in length, 

 while the upper side has decreased ; in other words, 

 the underside is in a state of tension and the upper 

 side in a state of compression. Manifestly, the 

 layers immediately subjacent to these outermost 

 layers will be slightly less extended and slightly less 

 compressed, respectively, than they were before the 

 weight was applied ; a median region of the bar 

 must, therefore, be neither extended nor compressed. 

 If neither stretched nor compressed, that region is 



