THE FOOT.
Bony structure of the foot.--Union of bones of tarsus;
astragalus and calcaneum (canalis tarsi, or tunnel of the
tarsus); astragalar articulations, inferior calcaneo-scaphoid
ligament; special functions of astragalus in the mechanism of
the foot.--The other articulations of the tarsus, and their
movements. _Metatarsus_ and _Metatarsal_ bones; importance of
fifth metatarsal with regard to form.--The _toes_ and their
_phalanges_.--Skeleton of the foot as a whole; form of the
foot.--Plantar arch.--_Proportions of the lower limb_; the foot
as a common measure of the lower limb and the height.
We will now examine briefly the bones of the _tarsus_, especially with
regard to the details which mark their articulations. The inferior
surface of the _astragalus_ presents two articular facets, separated by
a deep, oblique groove. These two facets correspond with two similarly
situated on the upper surface of the calcaneum. The astragalus thus
rests upon the calcaneum, but it is supported on the inner side by a
projection of the calcaneum, known by the name of the _sustentaculum
tali_; and the anterior facet for the astragalus is to be looked for
on the upper surface of this projection (Fig. 53, page 153). The
two facets of the calcaneum are also separated by a deep groove. It
follows, therefore, that when the astragalus is in its place, the
groove of the astragalus and the calcaneum meet and form a kind of
tunnel, which is called the _cavity of the tarsus_, or _canalis tarsi_.
This cavity is filled up during life by a strong ligamentous band,
which attaches the astragalus to the calcaneum, and is called the
_interosseous ligament_. Placed between the two calcaneo-astragaloid
articulations, one in front of it and the other behind, the ligament
forms a sort of pivot, around which the movements between the
astragalus and the calcaneum take place. It is round these joints as a
centre that the movements take place by which the foot as a whole is
turned inwards or outwards, and so that its outer and inner borders are
elevated.
We have next to consider the articulations of the calcaneum and
astragalus with the other bones of the tarsus. The articulation of
the anterior extremity of the calcaneum with the posterior surface of
the cuboid presents an interlocked arrangement which is surrounded
by strong ligaments, especially on its lower plantar surface (the
_inferior calcaneo-cuboid ligaments_, or _long and short plantar
ligaments_), so that between the calcaneum and the cuboid only a
slight gliding movement takes place, and to all intents and purposes
these two bones form one elastic piece. The plantar ligaments from
their strength have an important influence on the maintenance of
the arch of the foot. The cuboid bone articulates in front with the
metatarsal bones of the fourth and fifth toes, so helping to complete
the outer part of the contour of the foot. On the inner side of the
foot the bones completing the tarsus in front of the astragalus are
the _navicular_ (or _scaphoid_), and the three _cuneiform_ bones,
_internal_, _middle_, and _external_, which in turn are related to the
inner three metatarsal bones. The _astragalo-scaphoid articulation_
deserves the closest attention. The head of the astragalus, so called
from its prominent rounded shape, articulates with the navicular bone,
which is prolonged internally into a prominent _tubercle_. Between the
sustentaculum tali and this tubercle a very powerful ligament passes,
on which a part of the astragalus rests. The ligament is called the
_inferior calcaneo-scaphoid_ ligament, or _spring_ ligament, and by
its means an articulation is completed between the three bones--the
_astragalo-calcaneo-scaphoid_ joint. The whole weight of the body may
be supported by the inferior calcaneo-scaphoid ligament, which thus
serves an important purpose in supporting the arch of the foot. This
is the ligament which gives way in flat-foot. By these articulations
the foot possesses the power of lateral movement, by which the toes
are carried inwards or outwards, and the outer border of the foot is
elevated and the inner border is depressed, or the reverse. On the
other hand, the movements of flexion and extension take place chiefly
at the ankle-joint, the articulation, previously explained, of the
astragalus with the tibia and fibula.
[Illustration: FIG. 54.
RIGHT FOOT (internal border).--1, 2, 3, 4, calcaneum;--5, 6, 7,
astragalus;--8, posterior calcaneo-astragaloid articulation;--9,
head and neck of the astragalus;--10, 11, navicular or
scaphoid;--12, astragalo-scaphoid articulation;--13, 14,
internal cuneiform;--15, articulation of the internal cuneiform
with the scaphoid;--16, the articulation of the internal
cuneiform with the metatarsal bone of the great toe;--17, the
middle cuneiform;--18, its articulation with the scaphoid;--19,
the first metatarsal bone;--20, the second metatarsal bone;--21,
the articulation of the second metatarsal bone with the
cuneiform bones;--22, sesamoid bone;--23, 24, the phalanges of
the great toe; 25, 25, phalanges of the other toes.]
The navicular or _scaphoid_ bone articulates in front with the three
_cuneiform_ bones, and the cuneiform bones articulate with each other
by facets which are flat, but present rough surfaces towards their
inferior or plantar portions intended for interosseous ligaments.
Similar articulations exist externally between the scaphoid and
external cuneiform, and the cuboid bone. It is sufficient to say that
in all these articulations a slight gliding movement takes place, just
sufficient to give a certain elasticity to the plantar arch, which
they contribute to form. We may point out the peculiarities that are
to be noted in each of these bones: the _scaphoid_ or _navicular_ bone
is concave behind, convex in front, and is prolonged into a prominent
tubercle, which can be felt at the middle of the inner border of the
foot; the _cuboid_ is quadrilateral or pyramidal in form, and has an
oblique groove (14, Fig. 55) on its inferior or plantar surface, which
contains the tendon of the _long peroneal_ muscle (see below); finally,
the three _cuneiform_ bones are distinguished, counting from within
outwards, as the _internal_, _middle_, and _external_ cuneiform (Fig.
53, page 153); the middle cuneiform (6) is smaller than the others,
and does not extend so far forward, so that in relation to it the line
of the tarso-metatarsal joints forms a notch opening into the tarsus,
into which the posterior extremity of the second metatarsal bone is
received.
In front of the tarsus is found the _metatarsus_, corresponding to the
metacarpus of the hand, while the toes correspond to the fingers. We
have only a few words to say about those portions of the skeleton which
resemble the corresponding parts of the hand.
[Illustration: FIG. 55.
RIGHT FOOT (external border).--1 to 6, the calcaneum;--7, the
tibial surface of the astragalus;--8, the lateral facet of the
astragalus for the external malleolus;--9, the tunnel of the
tarsus (canalis tarsi);--10, the posterior calcaneo-astragaloid
articulations;--11, head and neck of the astragalus;--12,
the scaphoid;--13, cuboid;--14, commencement of the groove
on the inferior surface of the cuboid (for the tendon of the
long peroneus muscle);--16, external cuneiform;--17, middle
cuneiform;--18, the five metatarsal bones with their posterior
extremities (19);--20, 21, 22, 23, fourth, third, second, and
first metatarsal bones;--24 to 25, phalanges of the toes.]
The metatarsal bones, five in number, are long bones, each composed
of a prismatic shaft and two extremities, one posterior or tarsal,
more or less wedge-shaped; the other anterior or digital, forming a
rounded, globular head which articulates with the base of a phalanx.
The metatarsal bones are placed parallel side by side, but that of the
great toe is not markedly shorter than its fellows like the metacarpal
bone of the thumb, and the great toe does not possess anything like the
same amount of mobility as the thumb. It is further directed straight
forward so as to complete the straight line of the inner border of
the foot. With regard to particular details, it is necessary to note
in the first metatarsal (19, Fig. 54) its large size, in the second
(22, Fig. 55), its greater length, for it passes behind the others (in
entering into the notch which corresponds to the middle cuneiform), and
it also exceeds them in length, so that the second toe is as a rule a
little longer than the rest. In the fifth we must note the form of its
tarsal extremity, prolonged behind into a process (19, Fig. 55) which
appears as a prominence at the middle of the outer border of the foot,
and gives insertion to the _short peroneal_ muscle. Lastly, it may
be observed that all the metatarsal bones except the first are sloped
inwards and forwards, to point as it were towards the great toe.
The phalanges of the toes (Figs. 54 and 55) resemble in number and
arrangement the corresponding bones of the hand, only they are much
shorter, especially those of the little toe, the two last of which are
reduced to small osseous nodules, so that the fifth toe is always much
shorter than the rest. The names of the phalanges are the same as in
the hand (page 93).
_The Foot as a whole._--The skeleton of the foot forms an _arch_ which
presents two curves or concavities, one antero-posterior, the other
transverse. The _sole_ of the foot forms a hollow which extends from
the posterior extremity of the calcaneum to the anterior extremity of
the metatarsal bones; but this plantar hollow is much more elevated
on the inner side (Fig. 54) than on the outer side (Fig. 55). It is
necessary, in other words, in order to understand the general form of
the foot, to consider that its _dorsal surface_ looks upwards _and
outwards_, and the sole _downwards and inwards_. The _outer border_ is
thin, and comes almost in contact with the ground, its _inner border_
is thick and is raised from the ground.
The skeleton of a well-articulated foot placed upon a horizontal
surface comes in contact with the flat surface only by the posterior
extremity (_tuberosity_) of the calcaneum (heel), and by the heads
of the metatarsal bones (the balls of the toes, Fig. 52, page 151).
When the foot is covered with its soft parts these points of contact
are scarcely changed; for, except at the heel and the balls of the
toes, we see that the greater part of the outer border of the foot
touches the ground but lightly, unless the subject is carrying a heavy
load, which, pressing upon the plantar arch, brings its elasticity into
play and slightly flattens it. We shall mention later on the special
structures (ligaments and tendons) which act as cords which bend the
bow and maintain the plantar arch.
_Proportions of the Lower Limb._--As we have already inquired into the
ratio of proportions of the hand and upper limb, we shall now see if
the foot can furnish any measurements relative to the _proportions of
the body_. We find that we can make the foot, no more than the hand,
a common measure for the body in general and for the inferior limb in
particular. We must confine ourselves to such measurements as will
apply to the average subject. Thus it is easy to perceive upon the
skeleton that the distance from the upper margin of the head of the
femur to the inferior border of the internal condyle is equal to twice
the length of the foot; but this has no practical value--it cannot be
used on the living body, as it is difficult to recognise the level
of the upper part of the head of the femur. If, instead of the head
of this bone, we take the superior border of the great trochanter (a
part easily felt beneath the skin), we find that the length from the
superior border of the great trochanter to the inferior border of the
external condyle scarcely ever measures the length of two feet, as the
great trochanter is upon a considerably lower level than the head of
the femur.
The leg, including the thickness of the foot, is not as much as twice
the length of the foot--that is, the distance from the lower border of
the internal condyle of the femur to the ground (or the sole of the
foot); but it is interesting to observe that in general the length of
the leg, plus the thickness of the foot, is equal to the distance from
the great trochanter to the lower border of the external condyle; in
other words, the centre point of the lower limb (starting from the
great trochanter) corresponds exactly to the line of the knee.
When we compare the length of the foot with the leg, beginning from
below upwards, we find a regular proportion, and one of practical
interest--viz. that the line from the ground to the middle of the
patella usually measures twice the length of the foot.
As a common measure of the height of the body, the foot does not give
us a result that can be expressed by an even number. From the numerous
researches of Leger on this question, the length of the foot is
generally contained 6⅓ times in the total height. However, this number
presents an interesting fact when we express this proportion by taking
the third part of the foot for a unit; 6⅓ feet forming nineteen thirds
of the foot, we see that the height of the body contains nineteen
thirds of the foot. It is interesting to observe that the number 19 is
precisely that which expresses the proportion that the middle finger
bears to the height in the Egyptian canon, according to Charles Blanc.
With regard to the foot itself, we need only say that the
tarso-metatarsal line offers, on the skeleton, a simple means of
dividing the foot. This line is oblique from within outwards and
backwards; its inner extremity at the base of the first metatarsal bone
divides the foot into an anterior and posterior half, while its outer
extremity, at the base of the fifth metatarsal bone, divides it into a
posterior one-third and anterior two-thirds.