118 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 7.12. Microscopic structure shown in a cross section of human dried com- 

 pact bone, one complete unit of bony structure (Haversian system) and parts of 

 others. In life the central or Haversian canal (black) contains nerves, blood and 

 lymph vessels; the lacunae, also black, contain the bone cells. All nourishment and 

 oxygen come to the bone cells by way of the canals. Layers of bone surround each 

 canal like successive coverings of a cylinder. (Courtesy, Nonidez and Windle: 

 Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) 



and lymph vessels. The blood vessels of the Haversian canals are connected 

 with those in the marrow or with larger vessels entering and leaving the bone. 

 Thus, when young, even compact bone proves to be a living tissue through 

 which body fluids can circulate. Respiration occurs in bone cells and conse- 

 quently metabolism does also, the latter at a lower rate than in other tissues. 

 In the finest structure of the bone around the Haversian canal the fibers 

 are wound spirally and are thus made stronger as the fibers of rope are 

 strengthened by twisting. In spongy bone the "lattices" are like bridges which 

 increase the strength against blows and breakage. The intercellular substance 

 of bone acts as a storage for calcium and phosphorus. There is continual inter- 

 change of calcium between the blood and bones which keeps the calcium 

 content of the blood constant. Insufficient calcium and phosphorus cause 

 rickets, a softening of the bones. In small children this may be a cause of 

 bow-legs. 



Bone marrow is a soft cellular tissue in the central cavity of long bones 

 and the spaces of spongy bones. There are two closely related kinds, the 

 yellow and red. The yellow marrow that fills the central cavity of long bones 



I 



