354 PHYSIOLOGICAL DEVELOPMENT. 



placed for resisting the strains to be borne. So that both by 

 the increased quantity of dense matter and by its mechanically 

 more-advantageous position, the bendings of the bone are 

 progressively decreased. But as they are decreased, each 

 new layer formed on the concave surface has its substance 

 and its vessels less compressed; and the resulting growth 

 and induration are rendered less rapid. Evidently, then, the 

 additions, slowly diminishing, will eventually cease; and this 

 will happen when the bone no longer bends. That is to say, 

 the thickening of the bone will reach its limit when there is 

 equilibrium between the incident forces and the forces which 

 resist them. Here, indeed, we may trace with great clearness 

 the process of direct equilibration — may see how an unusual 

 force, falling on the moving equilibrium of an organism and 

 not overthrowing it, goes on working modifications until the 

 re-action balances the action. 



That, however, which now chiefly concerns us, is to note 

 how this marked adaptation supports the general argument. 

 Unquestionably bone is in this case formed under the influ- 

 ence of mechanical stress, and formed just where it most 

 effectually meets the stress. This result, not otherwise 

 explained, is explained by the hypothesis above set forth. 

 And when we see that this special deposit of bone is ac- 

 counted for by actions like those to which bone-formation in 

 general is ascribed, the probability that these are the actions 

 at work becomes very great.* 



* In recent years (since 1890) Prof. Wilhelm Roux, in essays on func- 

 tional adaptation, has set forth some views akin to the foregoing in respect 

 to the general belief they imply, though differing in respect of the physio- 

 logical processes he indicates. The following relevant passage has been 

 translated for me from an article of his in the Real-Encyclopadie der ge- 

 sammlen Heilkunde: — "A more complete theory of functional adaptation 

 by the author is founded on the assumption that the ' functional ' stimulus, 

 or ' the act of exercising the function ' (in muscles and glands), and espe- 

 cially, in the case of bones, the concussion and tension caused by stress and 

 strain, exert a ' trophic ' stimulus on the cells, in consequence of which, and 

 along with an increased absorption of nutriment, they grow and eventually 

 increase (or the osteoblasts at the point of greater stimulus form more bone) ; 

 while, conversely, with continued inactivity, by absence of these stimuli the 



