56 



HANDBOOK OF PHYSIOLOGY. 



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 between 



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 epiphy- 

 ses with the intermediate car- 

 /tilage be removed from a young 

 bone, growth in length is no long- 

 er possible; while the natural ter- 

 mination of growth of a bone in 

 length takes place when the epi- 

 physes become united in bony 

 continuity with the shaft. 



Increase in thickness in the 

 shaft of a long bone, occurs by 

 the deposition of successive layers 

 beneath the periosteum. 



If a thin metal plate be in- 

 serted beneath the periosteum of 

 a growing bone, it will soon be 

 covered by osseous deposit, but if 

 it be put between the fibrous and 

 osteogenetic layers, it will never 

 become enveloped in bone, for 

 all the bone is formed beneath the latter. 



FIG. 64. Transverse section of femur of a 

 human embryo about eleven weeks old. a. rudi- 

 mentary Haversian canal in cross section ; 6, in 

 longitudinal section; c, osteoblasts; d, newly form- 

 ed osseous substance of a lighter color ; e, that 

 of greater age ; /, lacunae with their cells ; g, a 

 cell still united to an osteoblast. (Frey.) 



Other varieties of connective tissue may become ossified, e g., the 

 tendons in some birds. 



Functions of Bones. Bones form the framework of the body; for 

 this they are fitted by their hardness and solidity together with their 

 comparative lightness; they serve both to protect internal organs in the 

 trunk and skull, and as levers worked by muscles in the limbs; notwith- 

 standing their hardness they possess a considerable degree of elasticity, 

 which often saves them from fractures. 



