THE GROWTH OF BONE 57 



around themselves just as a coral polyp does ; they 

 are included in the midst of the bone thus formed as 

 bone corpuscles. 



Increase in the length of the bone takes place by 

 the new additions at each end, where the layer of 

 cartilage between the shaft and the epiphysis is con- 

 stantly being transformed into bone ; but inasmuch 

 as its cells keep on dividing, the cartilage is not used 

 up in the process until the age of eighteen to twenty- 

 five is reached. It is usual for one epiphysis to unite 

 later than the other, and in that case the increase of 

 length is greater at this end than at the opposite, and 

 the nutrient artery to the shaft will be directed away 

 from the persistent epiphysis because the bone is, as 

 it were, pushed down inside the periosteum. 



So far, the results of recent investigation entirely 

 support and amplify the older opinion, A classical 

 experiment of John Hunter's may be quoted. He 

 inserted two leaden shot into the tibia of a young 

 pig, exactly two inches apart. When the animal had 

 grown up, he found that although the bone was of 

 course much longer, the shot were stiU exactly two 

 inches apart. Evidently, then, the increase of length 

 must have been at the ends, not by interstitial 

 increase of the shaft. 



More recently, Macewen has removed almost the 

 whole shaft of the right radius in a young dog by 

 the subperiosteal method, leaving the two ends. 

 After six weeks, there was strong and vigorous growth 

 from each epiphysis, and, aided by a bending of the 

 ulna, the two ends had come together, although no 

 periosteal growth of bone had taken place. One of 



