Part 45

a, M. flexor brevis pollicis; b, M. adductor pollicis; c, M. interosseus of second digit (M. flexor brevis digiti secundi); d, M. adductor digiti secundi; e, M. interosseus of third digit; f, M. interosseus of fourth digit; g, M. opponens digiti quinti; h, M. interosseus of fifth digit (M. flexor brevis digiti quinti); i, M. abductor digiti quinti. 1-5, the digits in order.]

Action.--Bend the digits toward the radial side.

B. MUSCLES OF THE THUMB.--=M. abductor brevis pollicis= (Fig. 77, w) (includes also the opponens pollicis of man).--A very minute, probably rudimentary muscle.

Origin from the transverse ligament (1) which connects the prominent fibrous, hairless projection that lies over the pisiform bone with the radial border of the head of the radius. The muscle passes radiodistad and ends in a very slender tendon, the insertion of which is into the base of the first phalanx of the thumb.

=M. flexor brevis pollicis= (Fig. 89, a).

Origin by fleshy fibres from the adjacent borders of the os magnum and scapholunar bones and from the fascia which bridges the intervening groove for the flexor carpi radialis.

Insertion by a short tendon into the base of the first phalanx of the pollex.

Action.--Flexor of the thumb.

=M. adductor pollicis= (Fig. 89, b).

Origin by fleshy fibres from the ventral border of the os magnum. It curves about the base of the first metacarpal on its ulnar side to its insertion.

Insertion by fleshy fibres into the base of the first phalanx of the pollex on its ulnar side.

C. MUSCLES LYING BETWEEN THE METACARPALS.--=Mm. interossei= (Fig. 89).--Small stout muscles lying on the palmar surfaces of the metacarpals of the second, third, fourth, and fifth digits.

Origin by fleshy fibres from the ventral or lateral surfaces of the bases of the metacarpals. Near the distal end of the metacarpal each divides into two masses which pass onto the lateral surfaces of the metacarpal, so as to leave the ventral surface of the distal end exposed.

Insertion partly onto the lateral surfaces of the base of the first phalanx and its sesamoids, and partly by a slender tendon (Fig. 84, h) which is continued dorsad to join the extensor tendon of the digit.

D. SPECIAL MUSCLES OF THE SECOND DIGIT (THE INDEX).--=M. flexor brevis digiti secundi= (or indicis).--This name may be applied to M. interosseus of the second digit (Fig. 89, c).

=M. abductor digiti secundi.=--This name might be applied to a portion of the interosseus of the second digit which is sometimes differentiated from the remainder.

Origin from the radial and ventral surfaces of the base of the second metacarpal, and from the ventral surface of the trapezium.

Insertion into the radial side of the base of the first phalanx of the second digit and into its sesamoid.

=M. adductor digiti secundi= (Fig. 89, d).

Origin from the ventral surface of the os magnum.

Insertion into the ulnar side of the base of the first phalanx of the second digit.

E. SPECIAL MUSCLES OF THE FIFTH DIGIT.--=M. abductor digiti quinti= (Fig. 89, i).--A thick, small, conical bundle.

Origin from the distal surface of the pisiform bone and from the transverse ligament on the ulnar side of the pisiform. It ends in a slender tendon which runs along the ulnar side of the fifth metacarpal to its

Insertion into the ulnar side of the base of the first phalanx of the fifth digit.

=M. flexor brevis digiti quinti= (Fig. 89, h; Fig. 88, e).--This covers the ventral surface of the fifth metacarpal, and is really the interosseus of this digit.

Origin by fleshy fibres from the ventral surface of the base of the fifth metacarpal; from the ventral process of the unciform bone, and from the transverse ligament proximad of the fifth metacarpal.

Insertion by fleshy fibres into the ventral border of the proximal end of the first phalanx of the fifth digit.

=M. opponens (adductor) digiti quinti= (Fig. 89, g).