68 HANDBOOK OF PHYSIOLOGY. 



From the foregoing description of the development of bone, it will be 

 seen that the common terms ossification in cartilage and ossification in 

 membrane are apt to mislead, since they seem to imply two processes 

 radically distinct. The process of ossification, however, is in all cases 

 one and the same, all true bony tissue being formed from membrane 

 (perichondrium or periosteum); but in the development of such a bone 

 as the femur, which may be taken as the type of so-called ossification in 

 cartilage, lime-salts are first of all deposited in the cartilage ; this calci- 

 fied cartilage, however, is gradually and entirely re-absorbed, being ulti- 

 mately replaced by bone formed from the periosteum, till in the adult 

 structure nothing but true bone is left. Thus, in the process of "ossi- 

 fication in cartilage," calcification of the cartilaginous matrix precedes 

 the real formation of bone. We must, therefore, clearly distinguish 

 between calcification and ossification. The former is simply the infil- 

 tration of an animal tissue with lime-salts, and is, therefore, a change of 

 chemical composition rather than of structure ; while ossification is the 

 formation of true bone a tissue more complex and more highly organ- 

 ized than that from which it is derived. 



Centres of Ossification. In all bones ossification commences at 

 one or more points, termed centres of ossification. The long bones, e.g., 

 femur, humerus, etc., have at least three such points one for the ossifi- 

 cation of the shaft or diaphysis, and one for each articular extremity 

 or epiphysis. Besides these three primary centres which are always 

 present in long bones, various secondary centres may be superadded for 

 the ossification of different processes. 



Growth of Bone. Bones increase in length by the advance of the 

 process of ossification into the cartilage intermediate between the dia- 

 physis and epiphysis. The increase in length indeed is due entirely to 

 growth at the two ends of the shaft. This is proved by inserting two 

 pins into the shaft of a growing bone: after some time their distance 

 apart will be found to be unaltered though the bone has gradually in- 

 creased in length, the growth having taken place beyond and not be- 

 tween them. If now one pin be placed in the shaft, and the other in 

 the epiphysis of a growing bone, their distance apart will increase as the 

 bone grows in length. 



Thus it is that if the epiphyses with the intermediate cartilage be 

 removed from a young bone, growth in length is no longer possible; 

 while the natural termination of growth of a bone in length takes place 

 when the epiphyses become united in bony continuity with the shaft. 



Increase in thickness in the shaft of a long bone occurs by the depo- 

 sition of successive layers beneath the periosteum. 



If a thin metal plate be inserted beneath the periosteum of a grow- 

 ing bone it will soon be covered by osseous deposit, but if it be put be- 



