78 THE ENGINES OF THE HUMAN BODY 



of the whole bone. There would be no need to rebuild 

 it. But we have already seen that growth in length takes 

 place at the disc under the head of the bone and along 

 the upper surface of the neck. While bone is being laid 

 down on the upper surface of the neck, it is necessary 

 that material be cut away from its lower surface, other- 

 wise the neck would become enormously thick and 

 unwieldy (fig. 22, B). This the bone-builders do ; while 



15 Year 



'5* Year 



A. B. 



Fig. 22. — A, The upper end of a thigh-bone at the fifth year, above which is 

 indicated the position which the head and neck will occupy when the fifteenth 

 year is reached. B, The form which the upper end of the femur would 

 assume if there was no remodelling. 



they lay down new material on one side of the neck, they 

 remove it at the other. The myriads of osteoblasts which 

 have charge of the upper end of the cylindrical shaft 

 have also a busy time, for the shaft is being expanded 

 upwards at the expense of the struts and ties which 

 ascend from it to strengthen the neck. Hence the whole 

 architecture of the neck has to undergo daily alterations ; 

 every strut and tie is ever being moved. Thus, as long 

 as the thigh-bone grows, its upper extremity is always 



