350 PHYSIOLOGICAL DEVELOPMENT. 



excess of exudation and hardening, occurring where the 

 strains are most intense, will form a cylinder having a dense 

 outside and a porous or hollow inside. These pro- 



cesses will be essentially the same in bones subject to more 

 complex mechanical actions, such as sundry of the flat bones 

 and others that serve as internal fulcra. Be the strains 

 transverse or longitudinal, be they torsion strains or mixed 

 strains, the outer parts of the bone will be more affected by 

 them than its inner parts. They will therefore tend every- 

 where to produce resisting masses having outer parts more 

 dense than their inner parts. And by causing most growth 

 where they are most intense, they will call out reactive forces 

 adequate to balance them. There are doubtless 



obstacles in the way of this interpretation. It may be said 

 that the forces acting on the outer layers in the manner 

 described, would compress the canals too little to produce 

 the alleged effects; and if evenly distributed along the whole 

 lengths of the layers, they would probably do so. But it 

 needs only to bend a flexible mass and observe the tendency 

 to form creases on the concave surface, to feel assured that 

 along the surface of an ossifying bone, the yielding of the 

 tissue when bent will not be uniform. In the absence of 

 complete homogeneity, the interstitial yielding will take 

 place at some points more than others, and at one point 

 above all others. When, at the weakest point the centre 

 of commencing ossification an extra amount of deposit has 

 been caused, it will cease to be the weakest; and adjacent 

 points, now the weakest, will become the places of yielding 

 and induration. It may be further objected that the hypo- 

 thesis is incompatible with the persistence of cartilage for so 

 long a time between the epiphysis of bones and the bony 

 masses which they terminate. But there is the reply that 

 the places occupied by this cartilage being places at which 

 the bone lengthens, the non-ossification is in part apparent 

 only it is rather that new cartilage is formed as fast as the 

 pre-existing cartilage ossifies ; and there is the further reply 



