BONE. 305 



any considerable development, it will be seen that the ossific 

 deposit takes place in the fibres of fibro-cellular tissue, inter- 

 mingled with which numerous granular and nucleated cells 

 occur ; these fibres, which are disposed in bundles, invariably 

 precede the deposition of bony matter, and mark out the 

 course of the future spiculse. (See Plate XXXIII. Jig. 3.) 



The granular cells just noticed have been observed by Dr. 

 Sharpey, who remarks upon their distribution in the direc- 

 tion of the future spiculse, and who considers that they are 

 connected in some way or other with the process of ossifica- 

 tion. 



There can scarcely be a question but that they are the 

 bone cells in a rudimentary state, their conversion into 

 which it is not difficult to trace. 



Now it does not appear that cartilage is at all concerned 

 in any one stage of the development of the parietal bone of 

 the human embryo. In that of the sheep and some other 

 animals a lamina of cartilage is present in connection with 

 the parietal bone ; but this takes no part in the process of 

 ossification, but merely serves as a support to the newly- 

 developed bone, and extends beneath the first-formed portion 

 of it alone. 



As the ossification advances still further, the interstices 

 between the first-formed spiculae become filled up, grooves 

 appear on the surface of the bone ; these radiate from the 

 centre towards the circumference, and ultimately become 

 converted into canals, which, being lined with a number of 

 concentric laminae, at length constitute complete Haversian 

 canals. The bone is then completely formed. 



Intra-cartilaginous Ossification. It has until recently 

 been supposed that the formation of bone always takes place 

 in cartilage ; this notion, as we have seen, is erroneous. 



Ossification is also usually described as the conversion of 

 cartilage into bone ; this idea will be presently shown to be 

 equally erroneous, and the fact demonstrated that the intra- 

 cariilaginous ossification does not differ essentially from the 

 mtra-membranous form. 



If the microscope be brought to bear upon a thin longitu- 



