THE INNER TISSUES OF ANIMALS. 335 



Already when considering the vertebrate skeleton under 

 its morphological aspect (§ 256) it was pointed out that the 

 formation of dense tissues, internal as well as external, is, in 

 some cases at least, brought about by the mechanical forces 

 to be resisted. Through what process it is brought about wo 

 could not then stay to inquire : this question being not 

 morphological but physiological. Answers to some kindred 

 questions have since been attempted. Certain actions to 

 which the internal dense tissues of plants may be ascribed, 

 have been indicated ; and more recently, analogous actions 

 have been assigned as causes of some external dense tissues 

 of animals. We have now to ask whether actions of the 

 same nature have produced these internal dense tissues of 

 animals. 



The problem is an involved one. Bones have more than one 

 stage: they are membranous or cartilaginous before they be- 

 come osseous ; and their successive component substances 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 liga- 

 ments holding it together, may be rudely compared to a 

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

 generally, th^ 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 that have each to bear a thrust in 

 the direction of its length, and partly of pieces that have each 

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