THE INNER TISSUES OF ANIMALS. 345 



so far differ that the effects of mechanical actions upon them 

 differ. And having to deal with transitional states in which 

 bone is formed of mixed tissues, having unlike physical 

 properties and unlike minute structures, the effects of 

 strains become too complicated to follow with precision. 

 Anything in the way of interpretation must therefore be 

 regarded as tentative. If analysis and comparison show that 

 the phenomena are not inconsistent with the hypothesis of 

 mechanical genesis, it is as much as can be expected. Let us 

 first observe more nearly the mechanical conditions to which 

 bones are subject. 



The endo-skeleton of a mammal with the muscles and 

 ligaments holding it together, may be rudely compared to a 

 structure built up of struts and ties; of which, speaking 

 generally, the struts bear the pressures and the ties bear the 

 tensions. The framework of an ordinary iron roof will give 

 an idea of the functions of these two elements, and of the 

 mechanical characters required by them. Such a framework 

 consists partly of pieces which have each to bear a thrust in 

 the direction of its length, and partly of pieces which have 

 each to bear a pull in the direction of its length; and these 

 struts and ties are differently formed to adapt them to these 

 different strains. Further, it should be remarked that though 

 the rigidity of the framework depends on the ties which are 

 flexible, as much as on the struts which are stiff, yet the ties 

 help to give the rigidity simply by so holding the struts in 

 position that they cannot escape from the thrusts which fall 

 on them. Now the like relation holds with a difference 

 among the bones and muscles : the difference being that here 

 the ties admit of being lengthened or shortened and the 

 struts of being moved about upon their joints. The mecha- 

 nical relations are not altered by this, however. The actions 

 are of essentially the same kind in an animal that is stand- 

 ing, or keeping itself in a strained attitude, as in one that 

 is changing its attitude — the same in so far that we have 

 in each a set of flexible parts that are pulling and a set of 



