72 THE ENGINES OF THE HUMAN BODY 



of these microscopic bone-builders has pushed its way 

 boldly into the very heart of the cartilage, its pathway 

 being marked by a trail of blood-vessels which keep it 

 supplied with the materials needed for future operations. 

 When we examine the colony and its surroundings under 

 great magnification we see that a kind of warfare is going 

 forward. The bone-builders have not entered a virgin 

 uninhabited territory. It is already occupied by builders of 

 a more primitive kind, one which builds with cartilage, the 

 original material used in the construction of levers in the 

 animal body. The bone-building colonists in the lower 

 end of the femur represent a more skilled kind of work- 

 man — a higher civilisation ; before them the cartilage- 

 builders fade away just as a primitive native race dies out 

 before the advancing wave of European colonists. The 

 bone-builders invade the homes of the cartilage corpuscles, 

 rebuilding them in that curious and strong form of 

 cement called bone. By the end of the first year another 

 colony has established itself within the cartilaginous head 

 at the upper end of the thigh-bone. 



When we turn to the stage of construction reached 

 about the fifth year of life (fig. 19) we see that the 

 bone-builders have been busy. They have added to the 

 shaft, making it thicker as well as longer ; the colony in 

 the lower end has spread outwards ; but the cartilage- 

 builders have also been busy, so that the lower end of the 

 bone is bigger in every way — both as regards its cartilage 

 and its bone. So, too, in the upper end of the thigh- 

 bone, the colony or " centre " in the head has increased 

 its dominion ; but the cartilage has grown as well. We 

 notice that two other colonies have invaded the cartilage 

 of the upper end — one in that outwork called the great 

 trochanter, and another in a less projecting prominence 

 named the lesser trochanter (fig. 19). By the fifteenth year 

 a further stage is reached in the construction of the lever 

 of the thigh. It is now three times the length it was at 

 birth. The colonies of bone-builders have invaded almost 

 the whole of the cartilaginous extremities, all save at certain 

 places. At each end of the shaft a layer of cartilage still 



