264 THE MECHANICAL FUNCTIONS. 



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 

 development as to have acquired a certain degree of soli- 

 dity and firmness, and to bear, as well as to require, the 

 support of more massive and rigid structures, this flexible 

 and elastic cartilage may be employed with great advantage 

 as its substitute. A hard and unyielding structure would, 

 in the early stages of its formation, have even been injuri- 

 ous. But in proportion as the fabric is enlarged, the ne- 

 cessity for mechanical support increases, and farther provi- 

 sion must be made for resistance to external violence. 



When, at length, all is prepared for the construction 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 scaffolding, the 

 whole must not be removed at once; each part must be car- 

 ried away, piece by piece, while the operation of fixing in 

 their position the beams and pillars of the edifice proceeds. 

 The way is cleared at first b}^ the absorption of the central 

 part of the cartilage, and a few particles of ossific matter 

 are deposited in its room. While this process is going on, 

 greater activity is displayed in the arteries; they rapidly 

 enlarge in diameter, so as to admit the colouring globules 

 of the blood; and they thus become visible to the eye, which 

 can now follow their course without difficulty. From being 

 at first red points, they soon spread out into lines, of which 

 we trace the branches to a certain extent, although we can- 

 not pursue them to their minuter ramifications. They now 

 assume more active functions, and hasten to execute their 

 task by depositing granules of calcareous phosphate: these 

 are laid down, particle by particle, in a certain determinate 

 order, and in regular lines, so as to foWn continuous fibres. 

 When a great number of these delicate fibres are gathered 

 together, and connected by other fibres, which shoot in va- 

 rious directions across them, a texture composed of an as- 

 semblage of long spicula, or thin plates, is constituted. 

 In the cylindrical bones, the spicula prevail, and they are 



