THE INNER TISSUES OF ANIMALS. J337 



of being displaced by it. Not forgetting that many processes 

 of 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 vertebra) 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 which the severest stress fulls. 

 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 straightncss when relieved, 

 again bends, usually 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 tact which here specially concerns us, 

 We have U&amp;gt; note that- IMC Mi-oiiirest tensions and pressures are 



