This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The hand contains not only the tendons of the long muscles which descend into it from the forearm, but also some short muscles. They may be divided into three sets, viz: a middle set, embracing the interossei and lumbricales; an external set, embracing the thumb muscles and forming the thenar eminence; and an internal set, embracing the little finger muscles and forming the hypothenar eminence.
The interossei muscles arise from the adjacent sides of the metacarpal bones; the lumbricales arise from the tendons of the flexor profundus digi-torum. They all insert into the fibrous expansion of the long extensor tendons at the sides of the proximal phalanges (Fig. 367). When they contract they flex the proximal phalanx and extend the middle and distal phalanges. The interossei have a second insertion into the sides of the base of the proximal phalanx. By their action the fingers may be separated one from the other, or approximated. When the fingers are straight the palmar interossei act as adductors, while the dorsal interossei act as abductors. 23
The thenar or thumb eminence has four muscles, the abductor pollicis, opponens, flexor brevis, and adductor. This latter is usually divided into two parts called the adductor transversus and adductor obliquus (Fig. 368).
The flexor brevis has two heads, an outer and an inner. The outer head is inserted into the base of the proximal phalanx on its outer side along with the abductor. The inner head, called by some the first volar interosseous, is inserted into the inner side along with the adductor; between the two heads runs the tendon of the long flexor of the thumb. The opponens inserts into the outer anterior border of the shaft of the first metacarpal bone.
The little finger, like the thumb, has abductor, opponens, and flexor brevis muscles, but no adductor. There is, however, a short muscle, the palmaris brevis, which is superficial to the palmar fascia and, passing transversely across the hypothenar eminence, inserts into the skin. It makes a dimple on the ulnar side when the hand is hollowed. The abductor and flexor brevis minimi digiti muscles insert on the ulnar side of the proximal phalanx, hence when they contract they tend to hollow the hand, as does also the opponens minimi digiti, which inserts on the ulnar side of the fifth metacarpal bone.
Fig. 367. - Showing the mode of insertion of the interosseous and lumbrical muscles.