BONE-BUILDERS 73 



persists, forming a partition between the lower end of the 

 shaft and the lower colony or epiphysis ; and another 

 between the upper end of the shaft and the colony of the 

 head (fig. 19). In these discs or partitions of cartilage 

 growth still goes on, and as the cartilage-builders lay 

 down new material the adjoining bone-builders invade and 

 occupy it. The cartilage partitions are constantly growing, 

 and thus the length of the bone is being steadily increased. 

 Growth in the length of the thigh-bone takes place in 

 these discs, but increase in the thickness of the shaft and 

 of the extremities is obtained by the deposition of new 

 strata on the surface of the bone. Then, when we look 

 at the thigh-bone as a finished lever (fig. 19) we notice 

 that the growth discs of cartilage have disappeared, and that 

 the end pieces of bone have become firmly annealed to 

 the shaft. After a struggle lasting twenty years — some- 

 times more, sometimes less, depending on the individual — 

 the cartilage-builders in the growth discs are vanquished 

 by the armies of bone-builders, which fuse together across 

 the cartilage chink, and thus the thigh-bone becomes a 

 solid lever from end to end. When that has happened 

 there can be no more growth in length ; our stature has 

 then reached its zenith. 



It is estimated that about two millions of bone-builders 

 are already engaged in the construction of the thigh-bone 

 of a newly born child ; by the time their task is com- 

 pleted an army of one hundred and fifty millions is em- 

 ployed on the task. Nor is this army demobilised when 

 growth is over ; it is maintained as a standing army to 

 look after the works and to efFect repairs. If, for example, 

 the thigh-bone should be broken, then the neigh- 

 bouring regiments of bone-builders are instantly summoned 

 to begin the laborious task of mending the breach. 



The growth discs are clearly very important as well as 

 very ingenious contrivances to allow bony levers to grow 

 in length without interfering with their daily work. It is 

 easy to see why they were not placed on the ends of 

 bones ; the upper end of the thigh-bone is kept con- 

 stantly moving in the socket of the hip-joint ; the lower 



