THE SUPPORTING TISSUES. 97 



it will be seen that bones grow in length really by the car- 

 tilaginous ends growing, and then the cartilage being 

 removed from the inner side in the manner just described. 



In this way not only has the entire cartilage of the shaft 

 been removed, but as the periosteum forms more and more 

 bone beneath itself osteoclasts begin to remove bone from 

 the medullary cavity side, and so this cavity becomes larger 

 and larger. The explanation of this probably consists in 

 the fact that the first bone deposited by the periosteum is 

 not as strong as that which is later on deposited, and so it is 

 removed in order to avoid surplus weight. This eating 

 away of the bone from the inner side continues for a long 

 time, and so it happens that the medullary cavity of an 

 adult bone may be very much larger than the piece of car- 

 tilage which originally filled it. From these statements it 

 is clear that the bone grows in thickness only by the addi- 

 tion of layers around the outside by the osteoblasts under 

 the periosteum. The older notion that a bone deposit also 

 takes place by osteoblasts which line the medullary cavity 

 no longer seems valid. 



The encroachment of the bone into the cartilage at the 

 end does not reach the ends of the bone as we see it in the 

 adult specimen, but after the cartilage has grown to the size 

 intended for the future bone, independent centers of ossifi- 

 cation start up in these ends with processes identical with 

 those just described. In this way the bony epiphyses, as 

 they are called, at the ends of most long bones arise. L,ater 

 on the cartilage between the epiphyses is gradually ab- 

 sorbed from both sides and the bone becomes continuous, 

 or, as we say, the bone becomes "knitted." This does not 

 take place until quite late. For instance, in the humerus 

 the lower epiphysis unites with the shaft about the seven- 

 teenth year; the upper epiphysis about the twentieth year. 

 In the femur the upper end is joined to the body of that 

 bone about the nineteenth year, and the lower end becomes 

 knitted about the twentieth year. In the fibula this knit- 

 ting is still later, occurring at the lower end about the 



