PROBLEMS OF MODERN SCIENCE 



invasion of the cartilage by the bone-forming 

 tissue ; the spread and methodical advance of the 

 bone-forming tissue into the cartilage ; the growth 

 and expansion of the cartilage at either end, 

 whereby the gradual increase in length and size 

 is attained ; the mutual interdependence of the 

 bone-building cells and the excavating or absorb- 

 ing cells, the latter of which are not only responsible 

 for attacking and absorbing the cartilage, but are 

 further responsible for excavating some of the de- 

 posited bone, and account for the appearance and 

 expansion of the central or marrow cavity and the 

 production of the spaces in the lattice-work forma- 

 tion. As the final result of these manifold activities, 

 not only are the tapering walls of the cylindrical or 

 tubular part of the bone a marvel of mechanical 

 proportion, but every one of the innumerable tiny 

 spicules of bone constituting the lattice-work is in 

 its proper place and position, is of the requisite thick- 

 ness and length. 



The cells concerned in the building of the 

 thigh-bone are to be numbered, not by hundreds 

 or by thousands, but by millions. If a census of 

 the cells concerned in the building of the whole 

 skeleton were taken, the figure would be too vast 

 to convey any intelligible impression of the multi- 

 tude engaged. The organisation whereby every 

 cell is in its right place, doing its proper amount 

 of work and at the right time, is such as to be 

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