18 GENERAL STRUCTURE OF THE BODY. 



nutrient material finds its way from the Haversian canals 

 to all parts of the bone. 



Within the bone is the medulla or marrow, which is of 

 two varieties: the yellow, which is largely fat and is found 

 in the long bones of adults, and the red, which is nearly 

 three-fourths water and is found in most of the other 

 adult bones and in the bones of the fetus and of the 

 infant. 



Lining the medullary and cancellous cavities is a 

 delicate connective tissue lining, the endosteum, which 

 contains many bone-forming cells, while on the outside 

 of the bone, except at the articular ends, is the perios- 

 teum with its outer protective layer and its inner vas- 

 cular layer containing osteoblasts or bone-forming cells. 

 The periosteum is essential for the growth of new bone 

 where the old bone has died, and if the periosteum is 

 removed from healthy bone the part beneath is liable to 

 die, as it is by the constant growth of the osteoblasts 

 that the bone grows and is renewed. In the repair of 

 broken bones tissue is formed between and around the 

 broken ends. 



Bone Formation. Most of the skull and face bones 

 begin as membranes of connective tissue, that is, are 

 formed in membrane. Bones are also formed in carti- 

 lage, the bone formation in this case beginning from 

 centers of ossification, where the deposit of lime salts in 

 the intercellular substance begins, the salts coming to 

 the centers dissolved in the plasma. Such a center of 

 growth in a bone is called the epiphysis and is separated 

 from the main part of the bone or diaphysis by cartilage 

 until full growth is attained, when ossification becomes 

 complete. So in surgery, in working on the bones of 

 children, part of the epiphysis should always be left for 

 the sake of future growth. The outer shell of compact 

 tissue is deposited by the periosteum. 



Chemical Composition of Bone. Chemically bone is 

 composed of about one-third organic or animal matter, 

 largely gelatine, and two-thirds inorganic matter, includ- 



