Part 54

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.