26 



Life and Health 



them. They are tunnels left in the compact substance of 

 the bone, through which run blood vessels for its nutri- 

 tion. The black bodies are minute cavities called lacuna, 

 while the fine lines are very minute canals, canaliculi, 

 which connect the lacunae and the Haversian canals. 



The Haversian canals contain tiny blood vessels, while 

 the lacunae contain bone cells which have made the hard 

 material of the bone and are ready to make 

 more of it if occasion should arise. Very 

 fine branches from these bone cells pass 

 into the canaliculi in growing bone. The 

 canaliculi are far too small to allow the en- 

 trance of blood cells. They admit the 

 passage of nothing but a thin liquid from 

 the blood, the "plasma," which nourishes 

 the bone and -keeps it in a state fit for self- 

 repair when injured by disease or violence. 

 FIG. 15. Longitu- /36. The Self-Formative Power of Bones, 

 dinai Section of Thus bones are not d lifeless substances, 



Bone. 



(The Haversian canals but are the very type of activity and change, 

 are seen branching j n ijf e ^ e y are not only richly supplied with 



and communicat- . . . . . . , r 



ing with one an- blood from the nutrient artery and from 

 other.) the periosteum covering the surfaces, but 



they also contain countless little cells of 

 living material which maintain an endless network through- 

 out their whole structure. 



Bones have, therefore, like all other living structures, 

 a self-formative power, and draw from the blood the mate- 

 rials for their own nutrition. 



In all respects bones are as much cared for as are the 

 softer tissues of the body, 



