CHAPTER VI. 
In the joint of the knee (Tab. xl.) arc two loolc, almoft annular carti¬ 
lages, which being thick at their outer edges, and thin at their inner ones, 
they make the greateft parts of the two fockets in this joint. The ufc of 
thefe cart] lages is to make variable lockets to fuit the different parts of the 
lower end of the os femoris; for none but a round head and a round cavity 
can fuit in motion, unlefs the fliape of one or the other alters; and it is 
plainly neceflary, that this lower end of the os femoris fhould be flattifh, 
and projected backward, to give advantage to the mufcles that extend the 
tibia, by letting the center of motion backward; which mechanifm, 
though it equally leffens the power of thofe mufcles which bend this 
joint, is yet of great fervice, becaufe the extending mufcles move this 
joint under the weight of the whole body, but the flexors onlyraife the 
legs; and as no head or locket moves fo eafily as round ones, here leems to 
be fome provifion made again!! the inconvenience of a flatti/li head and 
cavity, by having the fri&ion made upon two lurfaces, the os femoris up¬ 
on the loofc cartilages, and the loofe cartilages upon the tibia. This contri¬ 
vance is imitated by mecbanicks, where the friftion of the joints of any 
of their machines is great, as between the parts of hook-hinges of heavy 
gates, and between the male and female ferews of large vices, where they 
ufually place a loolc ring. 
There are other cartilages which ferve to give fliape to parts. Of this 
fort arc the ciliary cartilages at the edges of the eye-lids, the cartilages of 
the outer ears, and thofe which compofe the lower part of the nofc, which 
have this particular advantage in thefe places, that they fupport and fliape 
the parts as well as bones do, and without being liable to be broke; and 
to thefe might be added thofe of the larynx, but they do not belong pro¬ 
perly to the fceleton. 
Of the ligaments. Bones that are articulated for motion are tied to¬ 
gether by ligaments, to prevent their dillocating, which alfo lurround the 
joints to contain their lubricating mucus. The thickncfs and ftrength of 
thefe ligaments are proportioned to the a&ions of the fcveral joints, and 
their lengths are no more than fuflicient to allow a proper quantity of 
motion; but the forms of them are different according to the different 
a&ions of the fcveral joints. 
The bones of the limbs that move to all lides have ligaments like pur- 
fes, which arife from or near the edges of the lockets of the receiving 
bones, and are inferted all round the received bones a little below their 
heads. The beginnings of thefe ligaments, from the edges of the fockets 
of the fcapula and os innominatum are very hard, almoff cartilaginous, 
