54 



HAND-BOOK OF PHYSIOLOGY. 



than of structure; while ossification is the formation of true bone a 

 tissue more complex and more highly organized 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 

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

 or epiphysis. Besides these three primary centres which are always pres- 

 ent 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 



FIG. 58. A. Longitudinal section of a human molar tooth; c, cement; cZ, dentine; e, enamel: V T 

 pulp cavity. (Owen.) 



B. Transverse section. The letters indicate the same as in A. 



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 re- 

 moved 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 growing 

 bone, it will soon be covered by osseous deposit, but if it be put between the 





