ANATOMY 



are therefore not only concerned in excavating the 

 cartilage, but are further concerned in absorbing 

 bone. 



The stage is now reached where the middle part 

 of the shaft of the future thigh-bone is occupied 

 by bone surrounding a small cavity, the rest of 

 the shaft and the expanded extremities being still 

 cartilaginous. The bone-forming tissue thence- 

 forward spreads towards either end of the shaft, 

 attacking and eventually replacing the cartilage. 

 The cartilage can only defend itself by growing, 

 and this it does with rapidity, expanding in every 

 direction, but retaining its general shape. As the 

 cartilage grows, it presents an ever wider and 

 wider front to the invading bone-forming tissue. 

 Thus it is that the bone as a whole grows in size, 

 the cartilage expanding at either end and being 

 constantly attacked and replaced by the bone- 

 forming tissue. 



Of the two processes, the expansion of the 

 cartilage and the attack of the bone-forming 

 tissue, the latter is relatively the more energetic. 

 The time arrives when the cartilage of the shaft 

 disappears and is replaced by bone, the bony shaft 

 being occupied by a central cavity, which by the 

 activity of the excavating cells is constantly en- 

 larging, keeping pace with the bone-formation and 

 slowly creeping towards either extremity. 



When the bone-formation reaches the expanded 



227 



