No. 3.] 



A STUDY IN FOOT STRUCTURE. 



313 



takes but little of the thrust, and has a proportionately small area 

 on which to distribute it. But supposing the bones plastic and 

 flattening out on one another ac- 

 cording to the pressure between 

 them, this is just what should be 

 expected. The tendency of use 

 during the life of an animal would 

 be constantly on the line of adjust- 

 ment. To make the fact clearer, 

 suppose the plane <^ in a given in- 

 dividual to be more nearly perpen- 

 dicular to the axis of the bone. 

 In that case if the unciform above 

 has a corresponding surface, b is 

 under greater pressure than a ; but 

 plasticity being admitted, the whole / 



tendency of use will be to flatten ^ j 

 out the surface b, broaden it in \ / 

 proportion to a, until an adjustment "^x 

 is produced. But suppose while b 

 has this new angle the correspond- 

 ing unciform surface remains as 



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*4\ 

 2> 





/ 



■h 



'^ 



/^ 



."C 



IS 



I 



M 



Resolution by plotting of the thrust of metacarpal IV on metacarpal III and the 

 unciform. Parts given are the thrust of the metacarpal, which is let equal to its 

 volume, and the inclination of the surfaces obtained by measure. 



formerly. A region of special pressure is then produced at x. 

 Use again, having its effect through plasticity, would tend to 

 flatten out the surface at x, bring the whole surface to bear, and 

 produce an adjustment as before. Observe what an opposite 

 and destructive effect growth in the region of special impact 

 would have. From the following diagram the effects can be car- 



