THE LEG, ANKLE-JOINT, FOOT.
The bony structure of the leg.--_Tibia_ and _fibula_:
tibio-fibular articulation; absence of mobility in the fibula
(differences between the leg and forearm: between the foot and
hand, also in the quadrumanous monkeys). Inferior extremities
of these bones: _tibio-fibular articulation_; the malleoli:
comparison of the internal and external malleoli with regard
to length, situation, and form.--The skeleton of the foot:
_tarsus_, _metatarsus_, and _toes_.--Importance of the bony
structure of the tarsus and its influence on the form of the
foot: construction and arrangement of tarsus, posterior half:
anterior half.--Articulation of the leg with the foot; movements
of the foot.
The two bones of the leg are placed, as we have previously seen (page
135), parallel to each other, the _tibia_ on the inner side, the
_fibula_ on the outer side and somewhat behind (Fig. 49). The _tibia_
can be felt beneath the skin in its whole length. Its _shaft_ is
triangular in section, and therefore presents three surfaces and three
margins (1, Fig. 49): an internal surface covered only by the skin and
appearing superficially as a long flat surface, broader above where it
looks a little forward, and inclining directly inwards at its inferior
part, where it becomes continuous with the prominence of the inner
ankle or internal malleolus (Fig. 50, page 150). The external surface
is slightly concave in order to lodge the antero-external muscles of
the leg, of which the principal is the tibialis anticus; below, this
surface inclines forwards, following the course of the tibialis muscle,
which, from the antero-external region of the leg, is directed towards
the inner border of the foot (Fig. 50, page 150). The posterior surface
of the tibia is entirely covered by the strong and numerous muscles of
the posterior region of the leg. Finally, of the three margins of the
shaft of the tibia the anterior is particularly prominent, and is known
by the name of the _crest of the tibia_ or _shin_ (6, Fig. 49).
The _fibula_ appears superficially, as already seen, in the region
of the knee, at its upper end. Its lower end is also obviously
subcutaneous at the outer ankle, where it forms the _external
malleolus_ (9, Fig. 49). The shaft of the bone is long and slender, and
is prismatic in form. It is curved from above downwards and forwards,
and downwards and inwards. It serves the purpose of a strengthening
bar for the tibia, at the same time that it increases the area for
the attachment of the muscles of the leg. The shaft of the fibula is
surrounded on all sides by these muscles.
[Illustration: FIG. 49.
BONES OF THE LEG (front view).]
The two bones are separated throughout their entire length by an
interval called the _interosseous space_ (Fig. 49), broader above than
below, and filled up by a fibrous membrane (_interosseous membrane_),
which, passing from one bone to the other, still further increases
the area for attachment of the muscles of the leg. Above, the fibula
articulates with the postero-external surface of the superior extremity
of the tibia, and this superior _tibio-fibular articulation_ possesses
a very slight gliding movement, exercised chiefly through the action of
the biceps muscle in external rotation at the knee-joint. Below, the
fibula is attached to the corresponding part of the tibia by a strong
_interosseous ligament_, forming a symphysis, or _synarthrodial joint_.
This _inferior tibio-fibular_ articulation has hardly any mobility:
it only gives a certain amount of elasticity to the ankle-joint, into
which the foot is received. We see, therefore, that there is, with
regard to mobility, a great difference between the bones of the leg
and those of the forearm; in the forearm the radius is moveable on the
ulna, and can turn in such a manner as to cross the latter, and produce
the movements of pronation and supination of the hand. Between the
fibula and the tibia there is no movement of the kind; the foot is not
capable of any movement which may be compared to that which takes place
in the hand during pronation and supination. We may say that it is the
same with monkeys, in the class called quadrumana; they have not the
power of pronation and supination of the foot, which, from this point
of view, and also in every other respect, is properly speaking a foot
and not a posterior hand, as their ancient name of _quadrumana_ would
lead us to suppose.
_Ankle-joint._--By their junction the inferior extremities of the
tibia and fibula constitute an articular cavity, which forms the
ankle-joint, by their articulation with a bone of the tarsus--the
_astragalus_. This _tibio-fibular_ cavity possesses three sides, of
which two, the superior and internal, are formed by the tibia, and one
only, the external, by the fibula; the two lateral walls correspond
to the two osseous parts which form the prominences of the ankles and
which are known by the name of _malleoli_ (_malleus_, a hammer). They
are distinguished as the _internal or tibial_ and _external or fibular
malleoli_ (Fig. 49, page 147). As the internal ankle or malleolus (8)
is of a form and situation very different to that of the external (11),
it is important to note here the configuration of the bones by which
the subcutaneous prominences are explained.
The malleoli differ in their _level_, in their _situation_, and
finally in their _form_ (Figs. 50, 51). First, we see at a glance that
the external or fibular malleolus (11) descends much lower than the
internal malleolus (8). Second, with regard to the transverse plane
of the two malleoli, just as the shaft of the fibula is situated
behind and to the outer side of the tibia, so the same position is
maintained by the inferior extremities of the two bones, and the
external malleolus is on a plane posterior to the internal malleolus.
A transverse line which passes through the centre of the internal
malleolus, passes outwards in front of the anterior border of the
external malleolus; and, on the other hand, a transverse line, passing
through the centre of the external malleolus, passes inwards behind
the posterior border of the internal malleolus. Thirdly, with regard
to the differences of form, these are the direct result of the shape
of the osseous parts. The malleolar portion of the tibia, or internal
malleolus, is square, presenting a horizontal inferior border, and
two vertical borders--one anterior, the other posterior. On the
contrary, the malleolar portion of the fibula, or external malleolus,
is triangular in shape, or rather like the head of a serpent; it
terminates below in a pointed extremity, formed by the convergence of
the two oblique borders--one anterior, the other posterior--of which
the anterior is the more sloping.
[Illustration: FIG. 50.
OUTLINE OF ANKLE AND FOOT (inner side).]
[Illustration: FIG. 51.
OUTLINE OF ANKLE AND FOOT (outer side).]
Before entering into a study of the articulation of the leg with the
foot, or _ankle-joint_, we must glance at the bony structure of the
foot as a whole, so as to understand properly the significance of the
position of one of the bones (_astragalus_) in relation to this joint.
[Illustration: FIG. 52.
OUTLINES OF THE FOOT (A, Dorsum; B, Sole).]
Just as the hand is composed of three sets of bones--the carpus,
metacarpus, and fingers--so also the foot is composed of a similar
series--the _tarsus_, _metatarsus_, and _toes_; but while in the hand,
where the function is principally that of prehension, the fingers are
long and the carpus very short, in the foot, which serves as a base
of support, the toes are comparatively short, while the tarsus, which
corresponds to the carpus, is of considerable size; it forms, in fact,
one-half of the length of the foot. In order to understand the form of
the foot and its mechanism it is necessary to make a particular study
of the bones which compose the tarsus.
As the carpus in the hand is formed by two rows of bones, so also the
tarsus is composed of two groups. In the hand the carpal bones are
grouped in two more or less transverse rows. In the foot, on the other
hand, the rows of tarsal bones are longitudinally arranged; and the
inner row overlaps the outer row in relation to the back part of the
tarsus. There are two bones in the outer row: the _calcaneum_ or _os
calcis_ behind, which forms the prominence of the heel and rests on
the ground below: and the _cuboid_, articulating with it in front and
carrying anteriorly the two outer metatarsal bones and the two outer
toes. The inner row consists of five bones: (1) the _astragalus_ or
_talus_ behind, which articulates with the bones of the leg and helps
to form the ankle-joint above, which rests below on the upper surface
of the calcaneum, and which articulates in front with the _navicular_
bone; (2) the navicular or scaphoid bone; and in front of this the
three _cuneiform_ bones (_internal_, _middle_, and _external_), which
lie above and internal to the cuboid bone, and carry the three inner
metatarsal bones and the three inner toes.
[Illustration: FIG. 53.
THE BONES OF THE FOOT, seen from the dorsal surface.--1,
astragalus (with its head and neck, 2);--3, calcaneum;--4,
scaphoid;--5, internal cuneiform;--6, middle cuneiform;--7,
external cuneiform;--8, cuboid;--9, the metatarsus;--10, 11, the
two phalanges of the great toe;--12, 13, 14, the first, second,
and third phalanges of the other toes.]
After this brief sketch of the tarsus, and before entering into the
details of the configuration of its parts and the whole taken together,
having seen the particular place occupied by the _astragalus_, we
must study its articulation with the tibia and fibula. The part of the
astragalus which is received into the cavity between the malleoli, is
formed by the posterior three-fourths (1, Fig. 53) of the superior part
of the bone, separated from the anterior fourth by a narrow portion
called the neck (2, Fig. 53). This articular part is in the form of a
pulley, with the antero-posterior groove hardly perceptible, but the
lips are prolonged over the sides of the bone, and come in contact
with the corresponding parts of the internal and external malleoli. It
is readily seen (Fig. 53) that the articular surface of the astragalus
is considerably wider in front than behind. The same is seen in
the shape of the lower end of the tibia. The _tibio-astragaloid
articulation_ permits movement chiefly in the anterio-posterior
plane, namely, movement forward (flexion of the foot) and backwards
(extension). During flexion of the foot on the ankle the astragalus
is received in the malleolar cavity as in a vice, and the result is
that no lateral movement is possible. When, however, the foot is
extended, and the toes are pointed, the narrower part of the articular
surface of the astragalus comes into relation with the wider portion
of the inter-malleolar articular surface, and the result is that in
this position a certain amount of lateral movement of the foot at the
ankle-joint is permitted, aided, it is true, by gliding movements
taking place among the tarsal bones themselves. Of the movements of
flexion and extension, that of extension is the most free, as it may
be continued until the axis of the foot becomes continuous with that
of the leg, and here it is arrested by the meeting of the posterior
border of the joint with the projections on the posterior border of the
astragalus; but the movement of flexion by which the dorsal surface
of the foot is brought near the anterior surface of the leg is more
limited, for it is impossible to cause the foot to make with the leg an
angle less than forty-five degrees, opening upwards and forwards. This
is accounted for by the shape of the articular surfaces. In proportion
as flexion is produced, the larger part of the articular surface of
the astragalus is wedged into the malleolar cavity; thus the movement
of flexion is arrested, and the foot is fixed. We cannot carry flexion
further without bursting asunder the tibio-fibular joint, just as we
should split a piece of wood by driving violently into it a wedge
larger than the cavity to be filled.