94 MECHANISM OF THE BONES. 



secured by a solid pivot, or any thing absolutely in- 

 flexible, the limbs would be more endangered. In 

 extreme accidents the gristle will yield, and permit 

 the limb to slip, without breaking the bone. All the 

 remedy required is a smart extension of the cords ; 

 whereas, if a bolt would have been more secure, the 

 difficulty is, it would have been too secure ; since it is 

 often the case that, if the joint did not give way, the 

 bone must. 



A. " In considering the joints,' observes Dr Pa- 

 ley, ' there is nothing which ought to move our grati- 

 tude more than the reflection, how well they wear. A 

 limb shall swing upon its hinge, or play in its socket, 

 many hundred times in an hour for sixty years to- 

 gether, without diminution of its agility ; which is a 

 long time for any thing to last ; for any thing so much 

 worked and exercised as the joints are." 



T. Are there any remarkable varieties in the joints 

 of different animals ? 



A. There are fishes which have joints of which the 

 skeleton of man and animals generally furnish no exam- 

 ples. They are to be seen in the fin. The general 

 structure of the fin resembles a fan, being composed 

 of spines, or long slender bones, with a membrane be- 

 tween them, which when raised from the body and 

 spread out, answers the purpose of a paddle, and 

 sometimes of a weapon of attack or defence. When 

 employed for the latter purpose it evidently requires 

 as much firmness as possible. In the pectoral fin of 

 some fishes there is a curious provision by which this 

 object is effected. A mechanic might be challenged 



