60 THE GROWTH OF BONE 



of building up and breaking down, absorption and 

 new formation, are continually going on in its cells 

 and molecules. When it is irritated, as for instance 

 when a pin is driven into compact bone, absorption 

 takes place, and the pin loosens in the course of a day 

 or two ; when it is withdrawn, osteoblasts wander 

 into the track and fill it with new bone. Even so soft 

 an organ as the tongue helps to maintain the shape of 

 the jaw, and after a successful operation for cancer, 

 the lower teeth come in time to slope towards the 

 buccal cavity. The interstitial changes in bone are 

 affected by various toxins and internal secretions : 

 during rickets the osseous tissue is at first softened, 

 finally more compact ; the pituitary secretion causes 

 it to undergo hypertrophy. 



So much is known and admitted. The evidence 

 which enables Macewen to go further and to deny 

 any share to the periosteum as such, is as follows: — 



In a dog, a strip of periosteum a quarter of an 

 inch broad and two inches long was peeled up from 

 the radius, leaving the attachment to the epiphysis 

 intact. It was buried between muscles. 



Eight weeks later, there was no trace of bone 

 formation in the fibrous intermuscular band which 

 represented the periosteum. On the other hand, 

 there was a bony ridge outgrown from the area 

 whence it had been stripped up. So far then from 

 forming bone, the periosteum must have been pre- 

 venting the outgrowth of bone. 



In other experiments, a strip of periosteum was 

 excised and immediately implanted in the neck of the 

 same animal around the jugular vein. Usually it 



