NATURAL THEOLOGY, 137 



kept in a different state by the action to which it 

 is exposed. At all events, we have a great par- 

 ticular benefit, though arising from a general con- 

 stitution; 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 within the joint, so that the ends of the 

 bones, instead of working upon one another, w ork 

 upon the intermediate cartilages. Cheselden has 

 observed,* that the contrivance of a loose ring is 

 practised by mechanics where the friction of the 

 joints of any of their machines is great, as between 

 the parts of crook-hinges of large gates, or under 

 the head of the male screw of large vices. The 

 cartilages of which we speak have very much of 

 the form of these rings. The comparison, more- 

 over, shows the reason why we find them in the 

 knees rather than in other joints. It is an expe- 

 dient, w^e have seen, w^hich a mechanic resorts to 

 only wiien some strong and heavy work is to be 

 done. So here the thigh-bone has to achieve its 

 motion at the knee, with the whole weight of the 

 body pressing upon it, and often, as in rising from 

 our seat, with the whole weight of the body to lift. 

 It should seem also, from Cheselden's account, that 



* Ches. Anat. p. 13, ed. 7, 



