PROBLEMS OF MODERN SCIENCE 



manner, being ranged in rows about the scaffold- 

 ing furnished by the remnants of the cartilage. 

 Examining them under a microscope, one is re- 

 minded of a vast army of soldiers, each unit in 

 its appointed place engaged in its ordered business. 

 The bone-building cell is a most indefatigable 

 workman, literally putting every atom of its energy 

 into the business. It builds bony substance all 

 round itself until it is eventually entombed in the 

 results of its own activity, and a minute bony 

 nodule is formed. About the bony nodules thus 

 first laid down other bone-building cells range 

 themselves, repeat the process, and the bony tissue 

 extends by a process of concretion. Once the 

 bone-building cell is encased by the bony sub- 

 stance it has manufactured its energetic activities 

 cease, and, as far as we know, it plays for the rest 

 of its existence the more placid role of controlling 

 and maintaining the welfare of the bony material 

 in its near neighbourhood. Unlike the cartilage 

 cell, it never divides, and consequently the bone 

 cannot expand, like the cartilage, by interstitial 

 growth. 



The bone first laid down has a temporary 

 existence only. Occupying the central part of 

 the shaft, it soon disappears by being excavated 

 and absorbed by the excavating cells. As a result, 

 a small central cavity, the rudiment of the marrow 

 cavity, comes into existence. The excavating cells 

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