ANATOMY 



irregular remnants of the dying cartilage, these 

 remnants providing a scaffolding upon which the 

 bone is built. A third set, ' vessel-formers,' are 

 concerned in the appearance of a complicated 

 system of minute tubes or pipes linked up with the 

 vessels outside, the fluid, that is the blood con- 

 tained therein, serving to bring material necessary 

 for the working of the bone-builders, and to carry 

 away the excavators' waste. The scene, as it is 

 viewed under the microscope, is wonderfully like 

 a busy house-building, with excavators, scaffolding, 

 builders, supply-pipes, and sewers. The single 

 pipe system, serving, at one and the same time, 

 to bring the needed material and to conduct the 

 waste away, must make a sanitary engineer green 

 with envy. 



The bone-building cells are the active agents 

 by means of which the bone appears in place of the 

 cartilage, and it behoves us to know whence they 

 come. To make an honest confession, we have 

 no certain knowledge. Some authorities maintain 

 that the cartilage cells, set free by the excavators 

 from their spaces in the groundwork of the cartilage, 

 are, as it were, rejuvenated, and take on a new role, 

 that of building bone. Such an explanation is, 

 on the face of it, most unlikely. It is more prob- 

 able that the bone-builders come in from with- 

 out with the invading cells. If this suggestion be 

 granted, we should be further enlightened as to 



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