352 PHYSIOLOGICAL DEVELOPMENT. 



mechanical actions and re-actions. It was only on tracing 

 out the processes here at work, that there suggested itself the 

 specific interpretation of the normal process, as above set 

 forth. When, from constitutional defect, bones do 



not ossify with due rapidity, and are meanwhile subject to 

 the ordinary strains, they become distorted. Remembering 

 how a mass which has been made to yield in any direction 

 by a force it cannot withstand, is some little time before it 

 recovers completely its previous form, and usually, indeed, 

 undergoes what is called a " permanent set ; " it is inferable 

 that when a bone is repeatedly bent at the same time that 

 the liquid contained in its canals is poor in the materials 

 for forming dense tissue, there will not take place a propor- 

 tionate strengthening of the parts most strained; and these 

 parts will give way. This happens in rickets. But this 

 having happened, there goes on what, in teleological language, 

 we call a remedial process. Supposing the bone to be one 

 commonly affected a femur; and supposing a permanent 

 bend to have been caused in it by the weight of the body; 

 the subsequent result is an unusual deposition of cartilagin- 

 ous and osseous matter on the concave side of the bone. If 

 the bone is represented by a strung bow, then the deposit 

 occurs at the part represented by the space between the bow 

 and the string. And thus occurring where its resistance is 

 most effective, it increases until the approximately-straight 

 piece of bone formed within the arc, has become strong enough 

 to bear the pressure without appreciably yielding. Now 



this direct adaptation, seeming so like a special provision, 

 and furnishing so remarkable an instance of what, in medical 

 but unscientific language, is called the vis medicatrix natures, 

 is simply a result of the above-described mechanical actions 

 and re-actions, going on under the exceptional conditions. 

 Each time such a bent bone is subject to a force which again 

 bends it, the severest compression falls on the substance of 

 its concave side. Each time, then, the canals running 

 through this part of its substance are violently squeezed 



