THE SKELETON. 25 
STRUCTURE OF BONE. 
Every bone is completely covered except on its articu- 
lating surfaces with a tough membrane, the periosteum, 
which serves for the attachment of muscles, and the renewal 
of bony tissue in case of injury. The long bones contain a 
cavity, the medullary cavity, filled with marrow (Fig. 12). 
Fic. 14. Cross-secTIon oF Cat’s Femur. X 5. 
cp, Compact tissue; cn, cancellous tissue; en, endosteum; mc, medullary 
canal; pe, periosteum. 
This cavity is lined with endostewm, a membrane similar to 
the periosteum. 
The shaft of the long bone is composed mainly of com- 
pact bony tissue through which extend longitudinally inter- 
communicating microscopic channels, Haversian canals, for 
the conveyance of blood-vessels, nerves, and lymphatics. 
The lacune, or spaces for the bone cells during life, are 
arranged concentrically about the Haversian canals. The 
canaliculi, or processes of the lacunze, communicate with one 
another. At the extremities of the bones the place of the 
medullary canal is taken up by cancellous tissue (Fig. 13, 
d), the compact tissue being very thin. The flat bones have 
no medullary canal, but the diploé or cancellous tissue lying 
between the outer compact tissue has its spaces filled with a 
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