OSSIFICATION. 263 



§ 3. Formation and Development of Bone. 



But it is not enough to contemplate the purposes so admi- 

 rably answered by tliesc arrangements. Our curiosity can- 

 "iiot but be powerfully excited to learn what processes and re- 

 fined series of means are employed by nature to raise and to 

 perfect all these artificially contrived structures. It fortu- 

 nately happens that in this instance we are permitted to 

 penetrate a little farther than usual into the secrets of orsjanic 

 evolution: for the succession of changes can be better followed 

 by the eye in the slow development of the harder parts, than 

 in the quicker growth of mere yielding and expansible tex- 

 tures. The peculiar material, also, of which bone is formed, 

 is easily distinguished by its hardness, its whiteness, and 

 its opacity from the softer and more transparent animal sub- 

 stance with which it is intermixed. Hence we are allowed 

 an opportunity of observing the earliest stages of its deposi- 

 tion, and of accurately following the subsequent chano-cs it 

 undergoes. • 



The parts of the embryo animal, which are destined to 

 become bone, partake of the soft and gelatinous consistence, 

 which, at that early period, characterizes all the textures of 

 the body; and they can hardly, indeed, be distinguished 

 from the semi-fluid portions which surround them. In pro- 

 cess of time, when tlic vascular circulation of the blood has 

 been established, and the newly formed arteries have extend- 

 ed their branches over every part of the nascent organization, 

 those vessels which are appropriated to the task of forming 

 the bones, arrive at 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 conii)arcd with tiic adjacent jelly. It 



