THE INNER TISSUES OF ANIMALS. 337 



of being displaced by it. Not forgetting tliat many processes 

 0^ the bones have to bear tensions, we may then say that 

 generally, though by no means universally, bones are in- 

 ternal dense masses that have to bear pressures— pressures 

 which in the cylindrical bones become longitudinal thrusts. 

 Leaving out exceptional cases, let us consider bones as masses 

 thus circumstanced. 



When giving reasons for the belief that the vertebrate 

 skeleton is mechanically originated, one of the facts put in 

 evidence was, that in the vertebrate series the transition from 

 the cartilaginous to the osseous spine begins peripherally 

 (§ 257) : each vertebra being at first a ring of bone sur- 

 rounding a mass of cartilage. And it was pointed out that 

 this peripheral ossification is ossification at the region ot 

 greatest pressures. Now it is not vertebrse only that follow 

 this course of development. In a cylindrical bone, though 

 it is differently circumstanced, the places of commencing ossi- 

 fication are still the places on w^hich the severest stress falls. 

 Let us consider how such a bone that has to bear a longitu- 

 dinal pressure is mechanically affected. If the end of a 

 walking-cane be thrust with force against the ground, the cane 

 bends ; and partially resuming its straightness when relieved, 

 again bends, usuall}' towards the same side, when the thrust 

 is renewed. A bend so caused acts on the fibres of the cane 

 in nearly the same way as does a bend caused by supporting 

 the cane horizontally at its two ends and suspending a 

 weight from its middle. In either case the fibres on the con- 

 vex side are extended and the fibres on the concave side com- 

 pressed. Kindred actions occur in a rod that is so thick as 

 not to yield visibly under the force applied. In the absence 

 of complete homogeneity of its substance, complete symmetry 

 in its form, and an application of a force exactly along its 

 axis, there must be some lateral deflection ; and therefore 

 some distribution of tensions and pressures of the kind indi- 

 cated. And then, as the fact which here specially concerns us, 

 we have to note that the strono-est tensions and pressures are 



