336 THE MECHANICAL FUNCTIONS. 



are appropriated to the task of forming the bones, 

 are spread over the pulpy masses where their work 

 is to commence. As sculptors, before working upon 

 the marble, first execute a model of a coarser and 

 more plastic material, so the first business of these 

 arteries is to prepare a model of the future bone, 

 constructed, not with the same material of which 

 it is afterwards to consist, but with another, of a 

 simpler and softer nature, namely cartilage. In 

 every case, then, cartilage is first formed, and 

 becomes visible by its greater opacity when com- 

 pared wdth the adjacent jelly. It is an exact 

 representation, in miniature, of the bone, which is, 

 in due course, to take its place. It is evident that 

 until the other parts of the fabric have proceeded 

 so far in their developement as to have acquired 

 a certain degree of solidity and firmness, and to 

 bear, as well as to require, the support of more 

 massive and rigid structures, this fiiexible and elas- 

 tic cartilage may be employed with great advantage 

 as its substitute : for a hard and unyielding struc- 

 ture would, in the early stages of its formation, 

 have even been injurious. But in proportion as 

 the fabric is enlarged, the necessity for mechanical 

 support increases, and further provision must be 

 made for resistance to external violence. 



When, at length, all is prepared for the con- 

 struction of the bone, the next step to be taken 

 is the removal of the cartilage, which had been 

 erected as the scaffolding for the intended building. 

 But in taking down this scafi^blding, the whole 

 must not be removed at once ; each part must be 

 carried away, piece by piece, while the operation 

 of fixing in their position the beams and pillars 



