344 PHYSIOLOGICAL DEVELOPMENT. 



When any main channel for blood, leading to or from a 

 certain part of the body, has been rendered impervious, 

 others among the channels leading to or from this same part, 

 enlarge to the extent requisite for fulfilling the extra func- 

 tion that falls upon them : the enlargement being caused, as 

 we must infer, by the increase of the currents carried. 



Here, then, are facts warranting inductively the deduction 

 above drawn. It is true that we are left in the dark respecting 

 the complexities of the process. How the channels for blood 

 come to have limiting membranes, and many of them mus- 

 cular coats, the hypothesis does not help us to say. But the 

 evidence assigned goes far to warrant the belief that vascular 

 development is initiated by direct equilibration; though in- 

 direct equilibration may have had the larger share in establish- 

 ing the structures which distinguish finished vascular systems. 



§ 301. Of the inner tissues which remain let us next take 

 bone. In what manner is differentiated this dense substance 

 serving in most cases for internal support? 



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 we 

 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 o\ 

 animals. 



The problem is an involved one. Bones have more than 

 one stage. They are membranous or cartilaginous before they 

 become osseous; and their successive component substances 



