70 THE ENGINES OF THE HUMAN BODY 



occupied by a bone-builder or bone corpuscle, so small 

 that one thousand of them may be set out in a row or 

 rank within the space of an inch. An osteoblast not only 

 builds up its cell, but can alter it or take it down again 

 when that is found necessary. Thus every bone is built 

 up by myriads of microscopic masons or engineers who 

 are given permanent quarters in habitations of their own 

 erection. We speak of a city as being alive with in- 

 habitants and a honeycomb alive with bees, but when we 

 speak of the bones of our bodies as being alive we mean 

 more. For we find that when a bone-builder dies a 

 curious change also comes over the wall of his habitation, 

 which seems to indicate that it too dies, and therefore we 

 may suppose that every particle of a bone is really alive. 



There is one difficulty met with in the construction of 

 bony levers which men who make levers of steel or of 

 wood know nothing of. They make their levers and 

 engines full size at once ; their creations are adults ; they 

 are quite " grown-up " from the start. It is otherwise in 

 the case of bony levers, which begin to be laid down when 

 the human limbs are so small that they can scarcely be seen 

 with the naked eye. On this microscopic basis the bone- 

 builders begin their task of constructing the thigh-bone ; 

 for twenty years or more they labour at it, enlarging it in 

 definite directions, altering it, and all the time keeping 

 pace with fellow-workmen who are employed in building 

 up neighbouring muscles, arteries, veins, nerves, and skin. 

 It is plain that all of these builders must work in harmony, 

 otherwise chaos would result in the formation of the limb. 

 Before birth the task of bone-builders is comparatively 

 easy, for the unborn child is then more or less at rest, 

 floating in and supported by the waters of the womb. 

 Some time after birth, however, when the limbs come 

 into almost constant use, all the bony levers of the limbs 

 have still to be altered and extended. The bone-builders 

 cannot put up a notice, " This lever is out of use owing 

 to repairs " ; they have to carry on during the day when 

 all the machinery of the human body is going full speed, 

 as well as at night when it is slowed down. Under these 



