242
UNIT 2
Covering, Support, and Movement of the Body
7
the
sustentaculum tali
(sus
0
ten-tak
9
u-lum ta
9
le; “supporter
of the talus”) or
talar shelf
. Te tibia articulates with the talus
at the
trochlea
of the talus. Te remaining tarsals are the lateral
cuboid
, the medial
navicular
(nah-vik
9
u-lar), and the ante-
rior
medial
,
intermediate
, and
lateral cuneiform bones
(ku-
ne
9
i-form; “wedge-shaped”). Te cuboid and cuneiform bones
articulate with the metatarsal bones anteriorly.
Metatarsus
Te
metatarsus
consists of five small, long bones called
meta-
tarsals
. Tese are numbered I to V beginning on the medial
(great toe) side of the foot. Te first metatarsal, which plays an
important role in supporting body weight, is short and thick.
Te arrangement of the metatarsals is more parallel than that
of the metacarpals of the hands. Distally, where the metatarsals
articulate with the proximal phalanges of the toes, the enlarged
head of the first metatarsal forms the “ball” of the foot.
Phalanges (Toes)
Te 14 phalanges of the toes are a good deal smaller than those of
the fingers and so are less nimble. But their general structure and
arrangement are the same. Tere are three phalanges in each digit
except for the great toe, the
hallux
. Te hallux has only two, proxi-
mal and distal.
Arches of the Foot
A segmented structure can hold up weight only if it is arched.
Te foot has three arches: two
longitudinal arches
(
medial
and
lateral
) and one
transverse arch
(Figure 7.35)
, which account
for its awesome strength. Tese arches are maintained by the
interlocking shapes of the foot bones, by strong ligaments,
and by the pull of some tendons during muscle activity. Te
ligaments and muscle tendons provide a certain amount of
springiness. In general, the arches “give,” or stretch slightly,
when weight is applied to the foot and spring back when the
weight is removed, which makes walking and running more
economical in terms of energy use than would otherwise be
the case.
If you examine your wet footprints, you will see that the me-
dial margin from the heel to the head of the first metatarsal
leaves no print. Tis is because the
medial longitudinal arch
curves well above the ground. Te talus is the keystone of this
arch, which originates at the calcaneus, rises toward the talus,
and then descends to the three medial metatarsals.
Te
lateral longitudinal arch
is very low. It elevates the
lateral part of the foot just enough to redistribute some of the
weight to the calcaneus and the head of the fiFh metatarsal (to
the ends of the arch). Te cuboid is the keystone bone of this
arch.
Te two longitudinal arches serve as pillars for the
trans-
verse arch
, which runs obliquely from one side of the foot to
the other, following the line of the joints between the tarsals
and metatarsals. ±ogether, the arches of the foot form a half-
dome that distributes about half of a person’s standing and
walking weight to the heel bones and half to the heads of the
metatarsals.
Tarsus
Te
tarsus
is made up of seven bones called
tarsals
(tar
9
salz)
that form the posterior half of the foot. It corresponds to the
carpus of the hand. Body weight is carried primarily by the
two largest, most posterior tarsals: the
talus
(ta
9
lus; “ankle”),
which articulates with the tibia and fibula superiorly, and the
strong
calcaneus
(kal-ka
9
ne-us; “heel bone”), which forms the
heel of the foot and carries the talus on its superior surface.
Te thick
calcaneal
, or
Achilles
,
tendon
of the calf muscles at-
taches to the posterior surface of the calcaneus. Te part of the
calcaneus that touches the ground is the
calcaneal tuberosity
,
and its shelflike projection that supports part of the talus is
Medial longitudinal arch
Transverse arch
Lateral longitudinal arch
(a) Lateral aspect of right foot
(b) X ray, medial aspect of right foot
Figure 7.35
Arches of the foot.
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