66 THE GROWTH OF BONE 



The strongest evidence for the older view, that 

 bone is laid down by the periosteum, is provided by 

 cases of acute periostitis, where pus forming inside 

 the bone finds its way out between the shaft and the 

 periosteum, so that the latter is extensively stripped 

 up. In many cases, new bone begins to form under 

 the detached periosteum, outside the pus, and the 

 shaft usually necroses. 



Macewen explains this occurrence by declaring 

 that if the inflammatory mischief is not very acute, 

 vasodilatation takes place within the bone, and the 

 osteoblasts are carried out by the Haversian canals 

 to the loose areolar space under the periosteum, to 

 which fibrous membrane some of them adhere. 

 When this is stripped up later, these osteoblasts lay 

 down new bone, but those remaining on the shaft are 

 deprived of their blood-supply and therefore die. If 

 the inflammatory mischief in the centre of the bone 

 is very acute, the whole shaft may die, especially if 

 thrombosis occurs, and therefore no osteoblasts 

 escape, so that no new bone at all can be laid down 

 under the periosteum. This is by no means a rare 

 occurrence. 



In local periostitis, again, which should rather be 

 described as an osteitis, the bone-forming cells are 

 brought by the blood-stream to the loose areolar 

 tissue underneath the periosteum, and finding there 

 a line of least resistance, are able to lay down 

 young bone, and so produce a localized swelling, 

 marked out in a skiagram by a faint line of shadow 

 close to, and parallel with, the shaft. 



During operations for the removal of bone, great 



