68 
OF MECHANICAL ARRANGEMENT 
tendons generally restrain the parts from going farther 
than they ought to go in the plane of their motion. In the 
knee , which is a joint of this form, and of great importance, 
there are superadded to the common provisions for the 
stability of the joint, two strong ligaments which cross 
each other; and cross each other in such a manner, as to 
secure the joint from being displaced in any assignable di¬ 
rection. [PI. XI. fig. 2.] “I think,” says Cheselden, 
“that the knee cannot be completely dislocated without 
breaking the cross ligaments .”* We can hardly help com¬ 
paring this with the binding up of a fracture, where the fil¬ 
let is almost always strapped across, for the sake of giving 
firmness and strength to the bandage. 
Another no less important joint, and that also of the gin- 
glymus sort, is the ankle; yet, though important, (in order, 
perhaps, to preserve the symmetry and lightness of the 
limb,) small, and, on that account, more liable to injury. 
[PI. XI. fig. 4.] Now this joint is strengthened, i. e. is 
defended from dislocation, by two remarkable processes or 
prolongations of the bones of the leg: which processes form 
the protuberances that we call the inner and outer ankle. 
It is part of each bone going down lower than the other 
part, and thereby overlapping the joint: so that, if the 
joint be in danger of slipping outward, it is curbed by the 
inner projection, i. e. that of the tibia; if inward, by the 
outer projection, i. e. that of the fibula. Between both, it 
is locked in its position. I know no account that can be 
given of this structure, except its utility. Why should the 
tibia terminate, at its lower extremity, with a double end, 
and the fibula the same, but to barricade the joint on both 
sides by a continuation of part of the thickest of the bone 
over it ? | 
* Ches. Anat. ed. 7th, p. 45. 
t The most obvious proof of contrivance is the junction of the foot to 
the bones of the leg at the ankle-joint. The two bones of the leg, called 
the tibia and the fibula , receive the great articulating bone of the foot 
(the astragalus) between them. And the extremities of these bones of 
the leg project so as to form the outer and inner ankle. Now, when we 
step forward, and whilst the foot is raised, it rolls easily upon the ends 
of these bones, so that the toe may be directed according to the inequali¬ 
ties of the ground we are to tread upon; but when the foot is planted, 
and the body is earned forward perpendicularly over the foot, the joint 
of the leg and foot becomes fixed, and we have a steady base to rest upon. 
We next observe, that, in walking, the heel first touches the ground. If 
the bones of the leg were perpendicular over the part which first touches 
the ground, we should come down with a sudden jolt, instead of which 
we descend in a semicircle, the centre of which is the point of the heel. 
And when the toes have come to the ground we are far from losing the 
