BONE-BUILDERS 



7i 



difficult conditions we shall find that bone-builders carry 

 out remarkable feats of engineering. 



Of the two hundred bones which make up the skeleton, 

 we shall select the thigh-bone as an example for study ; 

 it is the longest and strongest lever in all the body. The 

 stage in construction reached by it at birth is shown in 

 fig. 19. From its upper to its lower end it measures 



At birth 



'Great 



trochanter 



Shaft 



Vessels 

 ILotver end at birth 



- 'Lower end 



2 5 Year 



Fig. 19 — In this figure the following sections of the thigh-bone at various stages 

 of growth are shown : — Section of the thigh-bone at birth to show its structure. 

 Its lower end enlarged to show the centre of ossification and trail of blood- 

 vessels. Section at the fifth year. Section at the fifteenth year. Section at 

 the twenty-fifth year. 



about 4 inches — less than a fourth of the length it will 

 ultimately grow to. Only the shaft is made up of bone ; 

 it is formed not as a solid, but as a hollow pillar ; its 

 upper end, which fits into the socket of the hip-joint, is 

 composed of pure gristle or cartilage ; its lower end, 

 meeting the tibia in the knee-joint, is also made up of 

 cartilage. Within the lower cartilage, at the lower end, 

 can be seen a spot of bone (fig. 19). Somehow a colony 



