OP THE BONES. 375 



the bone forms and augments; and that the bone, which is 

 highly vascular at the period of its formation, becomes after- 

 wards less and less so. As to the state in which the bony sub- 

 stance is deposited, it is under the fluid form, and its successive 

 hardening depends either upon the continual addition of a 

 greater proportion of earthy substance, or upon the absorption 

 of the vehicle which gave it its fluidity. Ossification does not 

 depend upon the deposition of the earthy substance in an or- 

 ganic tissue, but upon the simultaneous formation of a tissue 

 containing at once both the animal substance and the earthy 

 substance. 



The phenomena of ossification are different in the different 

 kinds of bones. 



593. Ossification takes place at a very early period in the 

 long bones, commencing in them from one to two months af- 

 ter conception, according to the bone. Before the commence- 

 ment of ossification, no cartilages are observed in them. It is 

 the same also with them at the commencement of ossification; 

 there then being observed only a mucilaginous substance be- 

 tween the osseous cylinders. These osseous cylinders are at 

 first thick and short, whence results that they may elongate 

 greatly before growing thick. They correspond to the point 

 at which the principal medullary artery is afterwards perceived. 

 At the commencement of the third month, there are perceiv- 

 ed cartilaginous extremities at the end of these elongated bony 

 cylinders. Do these issue by vegetation from the interior of 

 the canal? These cartilaginous extremities have the same con- 

 formation as the extremities are to have at a later period; they 

 ossify, as has been said, in treating of ossification in general. 

 Most of them only ossify at the centre, and then form epiphy- 

 ses, which remain a greater or less time distinct at the ends of 

 the bones. In some of them ossification goes on from the 

 commencement, by the extension cf the body of the bone, in 

 the centre of their cartilaginous mass. 



594. The broad bones of the skull begin to ossify between 



the sixtieth and seventieth days. The pericranium and dura 



mater are then very vascular. There exists between these 



two membranes a mucous substance, which is itself very 



49 



