THE INNER TISSUES OF ANIMALS. 347 



it is differently circumstanced, the places of commencing ossi- 

 fication are still the places on which 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 re- 

 lieved, 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 sup- 

 porting the cane horizontally at its two ends and suspending 

 a weight from its middle. In either case the fibres on the 

 convex side are extended and the fibres on the concave side 

 compressed. 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 strongest tensions and pressures are 

 borne by the outer layers of fibres. Now the shaft of a long 

 bone, subject to mechanical actions of this kind, similarly has 

 its outer layer most strained. In this layer, therefore, on the 

 mechanical hypothesis, ossification should commence, and here 

 it does commence commences, too, midway between the ends, 

 where the bends produce on the superficial parts their most 

 intense effects. But we have not in this place simply 

 to observe that ossification commences at the places of greatest 

 stress, but to ask what causes it to do this. Can we trace the 

 physical actions which set up this deposit of dense tissue ? It 

 is, I think, possible to indicate a "true cause" that is at work; 

 though whether it is a sufficient cause may be questioned. 

 We concluded that in certain other cases, the formation of 

 dense tissue indirectly results from the alternate squeezing 

 and relaxation of the vessels running through the part; and 

 the inquiry now to be made is, whether, in developing bone, 



