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HUMAN BIOLOGY 





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many minute cavities (Fig. 19). /// these cavities the bone 

 cells lie self-imprisoned in walls of stone ; for these cells 



have formed the bone by deposit- 

 ing limestone and phosphate of 

 lime around themselves. There 

 are minute canals (3, Fig. 19), 

 however, through which nourish- 

 ment comes to the cells. The 

 watery portion of the blood passes 

 through these small canals from 

 the blood vessels that flow through 

 the larger canals (1, Fig. 19). 

 Bone cells may live for years, al- 

 though some of the other cells of 

 the body live only a few hours. 



New cells to repair the tissues are 

 formed by subdivision of the cells, as 

 with the ameba. Unlike protozoans, 

 many-celled animals are mortal because 

 the outer cells prevent the deeper cells 

 from purifying themselves perfectly and 

 obtaining pure food and oxygen. Even 

 the arteries of an old man become hard- 

 ened by the deposit of mineral matter 

 which the body has been unable to ex- 

 crete. 



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Fig. 18. -Cartilaginous 

 Tissue. A thin slice highly 

 magnified. 



a, b, c, groups of cells; m, inter- 

 cellular substance. 



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The body is kept alive and 

 warm by burning, or oxidation. 

 One fifth of the air is oxygen gas. 

 We breathe it during every min- 

 ute of our existence. It is car- 

 ried by the blood to all the tis- 

 sues. Not one of the cells could 

 work without oxygen. Without it the body would soon be 

 cold and dead, for oxygen keeps the body alive and warm 



Fig. 19. — Bony Tissue. Thin 

 slice across bone, as viewed 

 through microscope. 



Larger blood tubes pass through 

 the large holes (i); the cavities 

 containing bone cells lie in cir- 

 cles, and are connected by fine 

 tubes (3) with the larger tubes. 



