INTRODUCTORY 



37 



interstitial growth. Associated with growth is a constant destruction, 

 or absorption, of useless bone structure by osteoclasts. 



We may consider briefly growth in length and growth in thickness. 



A long bone increases in length through growth at each end, where 

 the layer of cartilage separates the diaphysis from the epiphysis. 



As the bone develops from a very small primary point near the 

 middle of the diaphysis, the ossification gradually extends toward the 

 secondary points in the epiphyses, and would reach them and coalesce, 



FIG. 11. 



-^ N - EP/II/I i/fi \fni- 



Tgk-_ Greater Tro- 



Epiphysisfor 



LI sst r Trochan- 



ter. 



Shaft. 



chanter. 



Lower Extremity. 



Posterior Surface. Anterior /Surface. 



PLAN OF THE DEVELOPMENT OF THE FEMUR BY FIVE CENTRES. 



were the cartilaginous layers, the remnants of the primitive cartilage, 

 not actively producing on each side new cartilage which the extending 

 centres convert into bone. This activity of the cartilage layers is 

 most marked on the side toward the diaphysis : hence the principal 

 growth in length of a bone is in the shaft. These processes of growth 

 have been demonstrated by a simple experiment. Into a long bone of 

 a young animal two small pegs were inserted at given distances from 

 the middle of the shaft and allowed to remain for some years. The 

 animal was then killed, and examination of the bone revealed that the 

 pegs were still the same distance apart. 



