STRUCTURE OF BONE. 463 



hardened cells, each of them being the centre of all those 

 comprehended within the range of its own canaliculi. Each 

 of these soft germinal masses is the centre of attraction for 

 the proper nutriment of bone, and is the active agent in with- 

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

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

 of attraction 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 canali- 

 culi are undoubtedly 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 fissures in certain hard cells in vege- 

 tables, only appear at a late stage in the development of 

 bone. Each osseous corpuscle has its own system of canali- 

 culi, 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 

 constitution 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 anasto- 

 mose 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. 



