OSSEOUS SYSTEM. 175 



solid, and of the dimensions of the hollow cylinder; and 

 if smaller and solid, there would not be surface enough 

 for the insertion of muscles and aponeuroses. 



Q. How is this medullary canal affected in fractures? 



#. When callus is first formed, gelatine is so deposited 

 as to close the canal; but this gelatine is subsequently ab- 

 sorbed, and the continuity of the canal reinstated. 



Q. What purposes do the flat bones serve in the sys- 

 tem? 



*#. They are especially appropriated to form cavities. 



Q. Are the cavities usually formed of one or many 

 bones? 



*ft. Of many bones; and hence, in part, their solidity, 

 and the limits set to their fractures. 



Q. Why are fractures of all the bones more infrequent 

 in children than in adults? 



#. Because ossification being incomplete, the bones 

 yield rather than break. 



Q. What is the form of the flat bones? 



t#. They have a convex and concave surface. The 

 curve forms in age a bony arch; a mechanism powerful 

 for strength and protection. 

 . Q. At what part is a flat bone thinnest? 



.#. It is thinnest in the centre; the circumference being 

 thickest, forms a broader surface for union, and offers 

 points for muscular origin in some bones, as the spine of 

 the ilium. 



Q. Are there cells in the flat bones? 



#. There are; they are between the two layers. 



Q. What purposes in the oeconomy are attained by short 

 bones? 



//. The two great ones of solidity and mobility; hence 



