_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.
