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What is Who proposed the "trajectory theory of bone formation"? and explain in detail?
Meyer, an anatomist, and Culmann, a mathematician, in 1867 observed that the bony trabecula in the femur followed definite engineering principles to resist functional force and this is trajectory theory of bone structure. 
In 1870, Julius Wolff, the German physiologist, stated that external morphology and internal architecture of bone are directly proportional to the functional forces acting upon it. 
Ref: Textbook of orthodontics , Gowrishankar ed 1, page 103