IN THE HUMAN FRAME. 
71 
At the knee, the extremity of the thigh-bone is divided by a 
sinus or cleft into two heads or protuberances: and these 
heads on the back part stand out beyond the cylinder of 
the bone. Through the hollow, which lies between the 
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 to the leg.* Who led these 
vessels by a road so defended and so secured? In the joint 
at the shoulder, 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-vessels steal to their destination in the arm, in¬ 
stead of mounting over the edge of the concavity.| 
III. In all joints, the ends 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 most proper 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 fot 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 
jt fair to gtate 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. [PI. XI. fig. 3.] Cheselden has observed J that the 
* Cheg. An. p. 35. f lb. 80. * lb. p. 13. 
