BONES. 95 



to favor the sliding of the articular surfaces upon each other- 

 The extremities of the bones that concur to form an articulation 

 correspond by having their respective configurations reciprocal ; 

 they are, in general, one convex, the other concave 



13. The means of union between bones is by fibrous parts 

 which bear the name of ligaments. These are very strong bands 

 or species of cords which surround the articulation or joint, hold- 

 ing together the two bones by their extremities. 



14. The articulations present a great variety in the motions of 

 which they are susceptible. 



15. The bones are also very different in their forms, and on 

 account of this circumstance they are divided into long, short 

 and flat bones. 



16. The Jong bones are generally cylindrical, of considerable 

 size, and in the interior, hollowed into a canal filled with a 

 fatty matter called tnnrroir; this form, without injuring their 

 solidity, diminishes their weight. At their extremities these bones 

 are enlarged to afford a broader surface for the articulation. 



17. It is easy to perceive that, if the bones were in contact 

 by small surfaces, their union would have been less solid, they 

 would have afforded only an uncertain and insecure motion, and 

 their derangement would have been as common as it is now rare. 



18. About their middle, the long bones are formed almost en- 

 tirely of very compact substance ; but at their swollen extremi- 

 ties the^y are chiefly composed of a spongy substance, which is 

 not so heavy. It is these bones that form the solid frame work 

 of the limbs. 



19. Neither the short nor the flat bones have any cavity in 

 the interior. 



20. The short bones are formed almost entirely of spongy sub- 

 stance, which lessens their weight without diminishing their vol- 

 ume The chief use of the flat bones is to form the parietes of 

 cavities which afford protection to internal organs : they are not 

 however, insusceptible of motion ; they furnish points of attach- 

 ment to many muscles. 



13. What a>-e ligaments? 



14. Do the articulations pernvt of a variety of motions? 

 1 >. How are the bones divided ? 



16. What is the general character of the long bones? Where is the mar. 

 row found? 



17. VVhy are the long bones enlarged at their extremities ? 



18. In what res ect do the extremities and middle of the long bones 

 differ in structure ? 



1!). Have the short and the flat bones any internal cavities? 

 20. What is the character of the short bones? 



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