66 
OF MECHANICAL ARRANGEMENT 
or the other prevails, as either is adapted to the motion 
which is wanted: e. g. a mortice and tenon, or ball and 
socket joint, is not required at the knee, the leg standing 
in need only of a motion backward and forward in the 
same plane, for which a hinge-joint is sufficient; a mortice 
and tenon, or ball and socket joint, is wanted at the hip, 
that not only the progressive step may be provided for, but 
the interval between the limbs may be enlarged or contract¬ 
ed at pleasure. Now, observe, what would have been the 
inconveniency, t. e. both the superfluity and the defect of 
articulation, if the case had been inverted: if the ball and 
socket joint had been at the knee, and the hinge-joint at 
the hip. The thighs must have been kept constantly to¬ 
gether, and the legs have been loose and straddling. There 
would have been no use, that we know of, in being able to 
turn the calves of the legs before; and there would have 
been great confinement by restraining the motion of the 
thighs to one plane. The disadvantage would not have 
been less, if the joints at the hip and the knee had been 
both of the same sort; both balls and sockets, or both hin¬ 
ges: yet why, independently of utility, and of a Creator 
who consulted that utility, should the same bone (the 
thigh-bone) be rounded at one end, and channelled at the 
other ? 
The lunge-joint is not formed by a bolt passing through 
the two parts of the hinge, and thus keeping them in their 
places; but by a different expedient. A strong, tough, 
parchment-like membrane, rising from the receiving bones, 
and inserted all round the received bones a little below 
their heads, encloses the joint on every side. This mem¬ 
brane ties, confines, and holds the ends of the bones to¬ 
gether; keeping the corresponding parts of the joint, i. e. 
the relative convexities and concavities, in close application 
to each other.* 
For the ball and socket joint , beside the membrane al¬ 
ready described, there is in one important joint, as an 
additional security, a short, strong, yet flexible ligament, 
inserted by one end into the head of the ball, by the other 
* This membrane is the capsular , or bursal ligarjient , common to 
every movable joint. It certainly connects the bones together, but does 
not possess much strength: its chief use is to produce and preserve the 
synovia in the part where it is required. The security and strength of 
the hinge-joint depends on certain ligaments called lateral ligaments, and 
the tendons of those muscles which pass over it. In the particular in 
stance of the knee, from its being the largest joint in the body, there is, 
as we shall presently find, an additional contrivance to prevent dislocation. 
Paxton. 
