THE INNER TISSUES OF ANIMALS. 341 



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 everywhere to produce resisting masses 

 having outer parts more dense than their inner parts. And 

 by causing most growth where they are most intense, will 

 call out reactive forces adequate to balance them — forms and 

 thicknesses of bone offering resistances equal to the strains, 

 however numerous and varied. There are doubt- 



less 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 capillaries too little to produce 

 the alleged effects ; and if evenly distributed along the whole 

 lengths of the layers, they would probably be 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. At these weakest points, and especially at one, 

 the action on the capillaries will be concentrated. 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. And in pro- 

 portion as the layer becomes filled with unyielding matter, 

 the remaining compressible parts of it, and their contained 

 capillaries, will be more severely compressed. It may be 

 further objected that the hypothesis is incompatible with the 

 persistence of cartilage for so long a time between the 

 epiphyses of bones and the bony masses which they ter- 

 minate. But there is the reply that the places occupied by 

 this cartilage, being places at which the bone lengthens, the 



