170 THE THEORIES OF EVOLUTION 



functional stimulation can account for this phenom- 

 enon. 



The functional stimulation of the bone is the me- 

 chanical action it bears while resisting the strains 

 which, in the various motions, tend to destroy its 

 rigidity. It is along this line of resistance that the 

 stimulation is strongest; consequently it is in the 

 lamellae orientated in this direction that nutrition is 

 most active. These lamella? develop very actively 

 while those orientated differently thrive poorly and 

 become atrophied. The presence of cavities within 

 certain bones is due to the fact that the parts located 

 in the centre are not sufficiently affected by functional 

 stimulation to survive. 



Roux cites many observations in support of his view. 

 In the case of fractures imperfectly set and in which 

 the two ends of a broken bone do not dovetail prop- 

 erly, the lamella? assume a peculiar direction which 

 is along the line of greatest resistance. Another phe- 

 nomenon more striking yet is observable in fractures 

 when the bones have not become completely solidified. 

 Owing to the mobility of the two segments a pseudar- 

 throsis forms between them, that is, an articulation 

 with cartilage and ligaments located in a place where 

 heredity could not have predetermined such a forma- 

 tion. 



Other cases have been instanced, after Roux, of 

 cartilage disappearing where friction ceases and ap- 



