26 NOURISHMENT OF BONE. 



If they were of hard bony substance throughout, they 

 would be either very heavy and unnecessarily strong, or 

 else too slender to give surface enough for the attach- 

 ment of muscles and other organs. A given quantity 

 of material if arranged in the form of a tube will bear a 

 much greater weight than if it were made into a solid 

 .rod of the same length as the tube. For this reason, 

 iron pillars used in buildings to support ceilings and 

 floors, are hollow. To cast them solid would make them 

 much heavier without great increase of strength. 



4. How Bones are Nourished. When the humerus is in 

 the body, it is closely surrounded by a connective-tissue 

 membrane, the periosteum. This membrane is full of 

 blood which nourishes the bone by means of innumer- 

 able little channels passing into and branching all 

 through it. These channels are too small to be seen 

 without a microscope, but even the most close-grained 

 part of every bone is full of them. As long as the hume- 

 rus is growing thicker, the periosteum is making new 

 bone on its inner side. If this membrane is peeled off, 

 the bone dies. The parts of the articular extremities 

 (Q>, 7V, Cpl, Fig. 7) which meet other bones at the 

 shoulder and elbow-joints are covered by cartilage instead 

 of periosteum. 



5. The Chemical Composition of Bone. The dried bone 

 of a man in middle life, consists of two parts of mineral to 

 one part of animal matter. The minerals give the bone its 

 hardness and stiffness; they may be obtained separate 



4. With what is the humerus surrounded ? How does the perios- 

 teum nourish the bone ? What happens if it be peeled off? Where 

 is cartilage found instead of periosteum ? 



5. Of what does the dried bone of a middle aged man consist? 



