IN THE HUMAN FRAME. 71 



hind parts of these two heads, that is to say, under the 

 ham, between the hamstrings, and within the concave re- 

 cess of the bone formed by the extuberances on each side ; 

 in a word, along a defile, between rocks, pass the great vessels 

 and nerves which go the leg.* Who led these vessels by a 

 road so defended and so secured ? In the joint at the slioul- 

 der, in the edge of the cup which receives the head of the 

 bone, is a notch which is joined or covered at the top with a 

 ligament. Through this hole, thus guarded, the blood-ves- 

 sels steal to their destination in the arm, instead of mount- 

 ing over the edge of the concavity .t 



III. In all joints, the end of the bones, which work 

 against each other, are tipped with gristle. In the ball 

 and socket joint, the cup is lined, and the ball capped with 

 it. The smooth surface, the elastic and unfriable nature 

 of cartilage, render it of all substances the properest for 

 the place and purpose. I should therefore have pointed 

 this out amongst the foremost of the provisions which have 

 been made in the joints for the facilitating of their action, 

 had it not been alleged, that cartilage in truth is only 

 nascent or imperfect bone ; and that the bone in these 

 places is kept soft and imperfect, in consequence of a more 

 complete and rigid ossification being prevented from taking 

 place by the continual motion and rubbing of the surfaces. 

 Which being so, what we represent as a designed advan- 

 tage, is an unavoidable effect. I am far from being con- 

 vinced that this is a true account of the fact ; or that, if it 

 were so, it answers the argument. To me, the surmount- 

 ing of the ends of the bones with gristle, looks more like a 

 plating with a different metal, than like the same metal kept 

 in a different state by the action to which it is exposed. 

 At all events we have a great particular benefit, though 

 arising from a general constitution ; but this last not being 

 quite what my argument requires, lest I should seem by 

 applying the instance, to overrate its value, I have thought 

 it fair to state the question which attends it. 



IV. In some joints, very particularly in the knees, there 

 are loose cartilages or gristles between the bones, and with- 

 in the joint, so that the ends of the bones instead of work- 

 ing upon one another, work upon the intermediate cartila- 

 ges. Chesselden has observed,| that the contrivance of 

 a loose ring is practised by mechanics, where the fric- 

 tion of the joints of any of their machines is great; as be- 



^ Ches. An, p. 35. t lb. 30, X lb. p. 13. 



