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). 



FIG. 14. CROSS-SECTION OF CAT'S FEMUR. X 5. 



cp, Compact tissue ; en, cancellous tissue ; en, endosteum ; me, medullary 

 canal ; pe, periosteum. 



This cavity is lined with endosteum, 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 lacunce, or spaces for the bone cells during life, are 

 arranged concentrically about the Haversian canals. The 

 canaliculi, or processes of the lacunae, 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 diploe or cancellous tissue lying 

 between the outer compact tissue has its spaces filled with a 



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