xion, and forwards again when the limb is
extended; under normal conditions the lateral moves more freely than
the medial. While the limb is partly flexed, a slight degree of
rotation of the leg at the knee is possible, and during this movement
the cartilages glide from side to side, and the tibia rotates below
them.

Any abnormal laxity of the ligaments of the joint may render the
cartilages unduly mobile, so that they are liable to be displaced from
comparatively slight causes, and when so displaced it is not uncommon
for one or other to be torn by being nipped between the femur and the
tibia. It is convenient to consider these "internal derangements of
the knee-joint" separately, according to whether the meniscus is
merely abnormally mobile, or is actually torn.

#Mobile Meniscus--Displacement of Medial Semilunar Cartilage# (Fig.
86).--The _medial_ meniscus exhibits undue mobility much more
frequently than the lateral, and the condition is usually met with in
adult males who engage in athletics, or who follow an employment which
entails working in a kneeling or squatting position for long periods,
with the toes turned outwards--for example, coal-miners. The tibial
collateral ligament, and through it the coronary ligament, are thus
gradually stretched, so that the cartilage becomes less securely
anchored, and is rendered liable to be displaced towards the centre of
the joint during some sudden movement which combines flexion of the
knee with medial rotation of the femur upon the tibia, as, for
example, in rising quickly from a squatting position, or turning
rapidly and pushing off with the foot, in the course of some game such
as football or tennis. It may occur also from tripping on a loose
stone or slipping off the kerbstone.

[Illustration: FIG. 86.--Diagram of Longitudinal Tear of Posterior End
of Right Medial Semilunar Meniscus.]

What actually happens when the meniscus is displaced would appear to
be, that the combined flexion and abduction of the knee opens up the
medial side of the joint by separating the medial condyles of the
femur and tibia, and that the medial meniscus in its movement backward
during flexion slips under the femoral condyle and is caught between
it and the tibia. It may even slip past the condyle and into the
intercondyloid notch, and come to lie against the cruciate ligaments.

The mechanism by which this lesion is produced doubtless explains the
greater frequency with which the _lef