INTEROSSEUS MUSCLES 499 



glides over the tuberositj' of the fifth metatarsal, it frequently sends a second fasciculus to be 

 attached to this bone (abductor ossis metatarsi quinti) . A special fasciculus from the tuberositv 

 often constitutes the lateral margin of the muscle. 



Nerve-supply.— The nerve arises from the lateral plantar. It maj^ be distributed either 

 near the deep or the superficial surface of the muscle. The former appears to he the case when 

 the muscle is slightly developed. The chief intramuscular branches then extend across the 

 middle third of the constituent fibre-bundles near the deep surface. In case the calcaneo-meta- 

 tarsal bundles are well developed, the nerve enters the proximal margin of the muscle and its 

 chief branches extend across the middle third of the more superficial muscle-bundles, finally 

 terminating in the distal margin of the muscle. 



Relations. — It is ensheathed by the plantar fascia and the lateral intermuscular septum. 

 It lies superficial to the quadratus plantse (flexor accessorius), the opponens and flexor brevis 

 of the httle toe, the long plantar ligament, and the tendon of the peroneus longus muscle. 



The flexor digiti quinti brevis (fig. 421).— Origfin.— From the sheath of the peroneus longus, 

 the tuberosity of the cuboid, and (3) the base of the fifth metatarsal. 



Structure and insertion. — The fibre-bundles take a nearly parallel course, although the belly 

 is shghtly fusiform. They are attached by short tendinous bands to the base of the first 

 phalanx of the little toe, the capsule of the corresponding joint, and the aponeurosis on the 

 dorsal surface of the toe. 



Nerve-supply. — A branch of the superficial ramus of the lateral plantar nerve sends twigs 

 to the middle third of the plantar surface of this and the following muscle. 



Relations.^t is covered medially by the plantar fascia, laterally by the abductor of the 

 fifth toe. Medially it lies superficial to the third plantar interosseous muscle. 



The opponens digiti quinti. — This muscle arises from the sheath of the peroneus longus 

 and the tuberosity of the cuboid by a slender tendon which passes over the tuberosity of the 

 fifth metatarsal and gives rise to fibre-bundles which are inserted on the lateral surface of the 

 fifth metatarsal. 



Nerve-supply. — From branches of the nerve to the flexor brevis. 



Relations. — It is covered by the abductor of the fifth toe. 



Actions. — ^The abductor and flexor brevis abduct the little toe and flex the first phalanx. 

 They act as extensors of the second phalanx. The opponens serves to draw the little toe medi- 

 ally in a plantar direction. 



Variations. — The muscles of this group may be more or less completely fused. The abduc- 

 tor, in addition to the variations mentioned above, may send tendons to the third and fourth 

 metatarsals. The opponens is frequently missing. The abductor accessorius digiti quinti is 

 a rare muscle which arises from the lateral process of the tuber of the calcaneus and is inserted 

 into the lateral surface of the base of the first phalanx of the httle toe. 



e. The Interosseous Muscles (fig. 422) 



Two groups of interosseous muscles are recognised, a dorsal and a plantar. 

 The dorsal are the larger and fill the interspaces. The first two are inserted into 

 each side of the base of the first phalanx of the second toe; the third and fourth 

 into the lateral sides of the bases of the first phalanges of the third and fourth toes. 

 The plantar interossei lie on the medial side of the ventral surfaces of the third, 

 fourth, and fifth metatarsals, and are inserted each on the medial side of the base 

 of the first phalanx of the corresponding toe. In the hand the axis about which 

 the interosseous muscles are arranged passes through the middle finger, in the foot 

 through the second toe. The nerve-supply is from the lateral plantar nerve. 



The interossei dorsales. — Each of the three lateral dorsal interosseous muscles arises from 

 —(1) the sides of the shaft and the plantar surface of the bases of the metatarsal bones bounding 

 the space in which it lies; (2) from the fascia covering it dorsally; and (3) from fibrous prolonga- 

 tions from the long plantar hgament. The first has a similar origin except that it is attached 

 medially to the base of the first metatarsal and to a fibrous arch extending from the base to the 

 head. 



Structure.— The component fibre-bundles of each muscle are inserted bipinnately on a ten- 

 don which begins high in the muscle and becomes free near the metatarso-phalangeal joint. 



Insertion. — The first and second on each side of the base of the first phalanx of the second 

 toe. The third and fourth on the lateral side of the bases of the proximal phalanges of the third 

 and fourth toes. Each tendon is adherent to the capsule of the neighbouring joint. They 

 send no well marked processes to the extensor tendons, as do those of the hand. 



The interossei plantares. — Each plantar interosseus arises — (1) from the proximal third 

 of the medial plantar surface of the shaft and from the base of the metatarsal on which it Ues; 

 and (2) from expansions of the long plantar hgament. 



Structure and insertion. — The obliquely placed fibre-bundles are longer than those of the 

 dorsal interossei, and are inserted in a tendon which hes near the medial border of the muscle, 

 becomes free near the metatarso-phalangeal joint, and is inserted into a tubercle on the medial 

 side of the base of the first phalanx of the digit to which it goes. 



Nerve-supply. — From the deep branch of the lateral plantar nerve several rami are given 

 off for the interossei. The nerve of each muscle enters the plantar surface in the proximal 

 third. The interosseous muscles of the fourth interspace, however, are usually supphed by a 

 branch from the superficial ramus of the lateral plantar nerve. 



