BONE-BUILDERS 79 



being remodelled, but the bone-builders never lose their 

 design for a single moment. 



Bone-builders are the most economical of engineers. 

 They fashion all their levers and props so that they 

 obtain the greatest strength with the least possible outlay 

 of material. It is so important for the human body, as 

 indeed it is for every kind of machine, to be made as 

 light as is compatible with strength. When the bicycle- 

 maker uses iron tubes for the framework of his machine, 

 he merely copies the design used in the animal machine. 

 The bone-builders, too, know all about the " factor of 

 safety law." Their levers are made strong enough to 

 withstand a load which will carry ten times or more the 

 one they are called upon to bear in their usual work. 

 Yet occasionally a bone is broken, not by an accident or 

 fall, but by the muscles which normally act on it. I have 

 seen a humerus which was broken in the act of throwing 

 a hand-bomb, but such mischances are very uncommon. 



Mention has already been made of the manner in which 

 osteoblasts mend the breach in a broken bone. But even 

 if a bone only becomes bent, as bones are apt to do in 

 rickets, the osteoblasts, lying in the hollow of the bend, 

 immediately begin to throw out a buttress to strengthen 

 the bone on its weak side. Nay, at times they have to 

 take over the duties of an undertaker. Occasionally it 

 happens, as the result of a blow or as an effect following 

 disease, part of a bone dies. When this happens, the 

 bone-builders immediately surrounding the dead area set 

 to work and cut away all the bonds which unite the dead 

 area to the living lever and cast the dead piece out. It 

 is a slow process, and sometimes the surgeon, taking pity 

 on their distress, relieves them of their toil and frees 

 the dead part by the use of a chisel. Thus the lever- 

 builders of the human machine are designers worthy of 

 the closest study by young engineers as well as by 

 students of medicine. 



