OSSIFICATION. 377 



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 flexible and elastic cartilage may be em- 

 ployed with great advantage as its substitute. A 

 hard and unyielding structure 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 build- 

 ing. But in taking down this scaffolding, 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 of the edifice proceeds. The way is 

 cleared at first by 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 dis- 

 played 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 



