66 THE STRUCTURE AND ECONOMY OF BONE. 



drawing this from the vessels, and appropriating it, partly for tho 

 nourishment of the hard cells, each of which has a centre of at- 

 traction within itself, but more probably for the formation of new 

 calcigerous cells, as the old cells dissolve and their debris falls 

 back into the returning circulation. The canaliculi are undoubt- 

 edly the principal channels for the passage of nutriment from the 

 capillaries to the calcigerous cells and germinal centres. They 

 are necessary in a hard texture, and like similar canals and fis- 

 sures in certain hard cells in vegetables, only appear at a late 

 stage in the developement of bone. Each osseous corpuscule has 

 its own system of canaliculi, these extending, for the purpose of 

 communicating with others, to the confines of its own territory ; 

 that is, to the boundaries of the space which was at one time 

 contained within the sphere of the primary cell of which it was 

 the nucleus. 



The accessory parts of the osseous texture, are the vessels 

 nerves, membranes, and oil. For my present purpose it is only 

 necessary for me to allude to the membranes, as one of them, the 

 periosteum, has been held to play a most important part in the 

 formation and economy of bone. 



The periosteum is not so important an element in the consti- 

 tution of a bone as has usually been supposed. In the adult 

 bone, it is nothing more than the fibrous sheath of the organ, 

 similar to the bounding or limiting membrane of other organs, 

 and in which the vessels ramify sufficiently to anastomose with 

 those of the comparatively few haversian canals which open on 

 the external surface. In the foetus it is much more vascular, 

 the external surface of the bone being at that period actively 

 engaged in growth. 



There exists in every true bone, a membrane or layer of much 

 greater importance, and infinitely more extended than the peri- 

 osteum. Between the blood-vessels and the walls of the haver- 

 sian canals, there is a layer of cellular substance. This cellular 

 substance is the product, its cells being the descendants of the 

 corpuscules of the cartilage or matrix in which the bone was 

 originally formed. It forms a blastema, originally produced 

 round each cartilage corpuscule by developement into a linear 

 series perpendicular to the ossifying surface : each of the secon- 



