A. MUSCLES ON THE DORSUM OF THE FOOT.--=M. extensor brevis digitorum=
(Fig. 90, _u_).--A broad thin muscle covering the tarsus and part of the
metatarsus on the lateral side of the dorsum of the foot. It may be
divided into three slips.
_Origin_ from the distal border of nearly the whole calcaneal annular
ligament (5), and from the proximal end of the dorsal surface of the
three lateral metatarsals. At the middle of the metatarsals the muscle
ends in three flat tendons which pass into the three interspaces between
the four tendons of the extensor longus. Each tendon is divided into two
branches.
_Insertion._--The lateral branch of each tendon is inserted into the
cartilaginous plate which lies in the metatarsophalangeal articulation
of the digit on the outer side. The medial branch joins the lateral side
of the extensor longus tendon on the dorsum of the first phalanx.
_Action._--Extensor of the digits.
B. MUSCLES OF THE SOLE OF THE FOOT.--=M. flexor brevis digitorum=
(=pedis perforatus=).--This is the direct continuation of the tendon of
the plantaris. It lies immediately beneath the superficial plantar
fascia. The flat belly of the muscle has its medial border attached by
an oblique tendinous band to the medial surface of the scaphoid and the
medial cuneiform. It divides into four slips which diverge to the four
toes, each ending in a flat tendon. The slips decrease in size from the
lateral to the medial side. The three lateral ones overlap one another
proximally.
_Insertion._--Each tendon expands at the distal end of the metatarsal
and is wrapped about the tendon of the deep flexor which perforates it.
The two halves unite beneath the perforating tendon and pass together
with the perforating tendon through fibrous rings, the annular
ligaments. One of these is attached to the sesamoids at the base of the
phalanx. The other is attached to the head of the first phalanx. The
tendons are finally inserted into the bases of the second phalanges.
The two annular ligaments are connected by fascia so as to form a
continuous canal for the tendons. This canal is lined by a synovial
membrane. Covering this canal is a more superficial layer of fascia.
Before passing into the proximal annular ligament the two middle tendons
unite each with the tendon of the corresponding lumbrical muscle. Each
also gives off a branch, the lateral one on its lateral side and the
medial one on its medial side. These branches, like those of the two
middle ones, are inserted into the common phalangeal fascia.
_Action._--Flexor of the second phalanges of the digits.
=M. quadratus plantæ= (called also the plantar head of the flexor longus
digitorum) is a thin flat muscle which takes _origin_ from the dorsal
part of the lateral surface of the calcaneus and the cuboid. Its fibres
converge to a flat tendon which passes transversely across the flexor
longus digitorum and beneath the flexor brevis digitorum and
calcaneometatarsal.
_Insertion._--Into the medial part of the outer surface of the tendon of
the flexor longus digitorum.
_Action._--It holds the flexor longus tendon in place.
=Mm. lumbricales.=--These are six.
_Origin._--The three larger ones have origin from the outer surface of
the expanded portion of the flexor longus digitorum on its distal half.
Each ends in a slender tendon.
_Insertions._--The tendons unite with the divisions of the tendon of the
flexor brevis which pass to the three lateral digits, at their entrance
to the first annular ligament. The one to the fifth digit is sometimes
absent.
The other three have _origin_ from the tendon of the flexor longus
digitorum at the point where it divides. Each occupies one of the three
intervals between the four divisions, and its fibres originate from the
proximal ends of the two tendons which bound its interval.
_Insertion_ by a very slender tendon into the medial side of the first
phalanges of the third, fourth, and fifth digits near their bases.
_Action._--Move the third, fourth, and fifth digits toward the medial
side.
=Mm. interossei.=--The interosseus of the second digit forms five
portions, which cover the ventral surface of the digit.
_Origin._--(1) By two tendons from the ventral process of the lateral
cuneiform. The muscle divides into four parts, three of which pass to
the medial side of the digit and act as abductors. They are called the
long, short, and middle abductors (abductor longus, brevis, and
intermedius digiti secundi).
_Insertions._--The short head into the medial sesamoid of the
metatarsophalangeal joint, the middle into the medial side of the base
of the first phalanx, and the long into the extensor communis tendon
near the distal end of the first phalanx. The names long, short, and
middle refer to the lengths of the tendons as determined by their points
of insertion.
The _fourth_ portion which comes from the lateral cuneiform has its
_insertion_ into the lateral side of the extensor tendon near the distal
end of the first phalanx. It is therefore an adductor longus digiti
secundi. In some cases there is an adductor brevis from this same head.
(2) The fifth part of the interosseus of the second digit has _origin_
from the middle of the ligament covering the peroneal canal, along with
the middle adductor of the fifth digit. The two muscles diverge.
_Insertion_ into the outer side of the base of the first phalanx of the
second digit. It is an adductor medius digiti secundi.
The interossei of the third and fourth digits are alike.
_Origin_ from the ventral surface of the base of the metatarsal. The
muscles cover the ventral surface of the shaft. At the head each divides
and passes onto the sides of the metatarsal.
_Insertion._--Each half of the muscle shows a tendency to divide into
two parts. One of these, the more superficial and ventral, ends in a
tendon which joins the extensor tendon near the distal end of the first
phalanx. The other part is inserted by short tendon- and muscle-fibres
into the side of the base of the first phalanx; one of the parts is
therefore a middle, and the other a long, adductor or abductor, the
adductores and abductores digiti tertii and quarti.
The fifth digit has five short muscles.
=M. abductor medius digiti quinti.=
_Origin_ from the ventral surface of the calcaneus and from the fifth
metatarsal.
_Insertion_ by a slender tendon into the lateral side of the base of the
first phalanx of the fifth digit.
=M. adductor medius digiti quinti.=
_Origin_ with the adductor medius digiti secundi.
_Insertion_ on the inner side of the base of the phalanx of the fifth
digit.
=M. opponens digiti quinti.=
_Origin_ by a flat tendon from the middle of the ligament which covers
the peroneal canal. It passes obliquely outward parallel to the last.
_Insertion_ on the inner side of the shaft of the fifth metatarsal.
The two remaining muscles of the fifth digit take _origin_ from the
outer part of the ligament covering the peroneal canal. They cover the
ventral surface of the shaft.
_Insertion._--The lateral one is inserted into the lateral sesamoid and
is therefore an abductor brevis. The medial one is inserted into the
extensor tendon and is therefore an adductor longus.
C. MUSCLES OF THE TARSUS.--=M. calcaneometatarsalis= (part of M.
adductor minimi digiti?).--A weak muscle made up largely of
tendon-fibres, some of which pass directly from origin to insertion. It
may be regarded as a ligament.
_Origin_, the lateral and ventral surface of the calcaneus near the
proximal end. It passes distad and slightly laterad outside of the
quadratus plantæ.
_Insertion_, the lateral side of the base of the fifth metatarsal, and
the adjacent surfaces of the cuboid and calcaneus.
_Action._--Probably causes slight motion of the cuboid on the head of
the calcaneus.
=M. scaphocuneiformis.=--A small muscle lying on the sole of the foot in
the depression between the lateral cuneiform, medial cuneiform, and the
lateral tubercle of the scaphoid. It is hidden by the overlying
ligaments.
_Origin_, the lateral tubercle of the scaphoid bone. It passes distad
and laterad.
_Insertion_, the lateral surface of the medial cuneiform.
_Action._--Rotates the medial cuneiform on the scaphoid and would thus
act as an opponens of the great toe if the great toe were present.