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 fingers are moved by two sets of muscles, a long set arising from the forearm and a short set which is confined to the hand. At present we are concerned only with the long extensors and flexors which are found in the forearm.
The flexors of the fingers consist of three separate groups of muscles: (1) the flexor profundus digitorum, and flexor longus pollicis, which insert into the distal phalanges; (2) the flexor sublimis digitorum; (3) the palmaris longus which, spreading out into the palmar fascia, is attached to the heads of the metacarpal bones and blends with the capsules of the metacarpophalangeal joints. It is an additional perforated flexor muscle (Fig. 322).
1. The flexor profundus digitorum is composed of four slips, one for each finger, and the flexor longus pollicis (Fig. 323) is a fifth slip that supplies the thumb. The flexor profundus arises from the anterior surface of the ulna and interosseous membrane while the flexor longus pollicis arises from the anterior surface of the radius and interosseous membrane. Their tendons pass through slits in the flexor sublimis digitorum opposite the proximal phalanges to insert into the bases of the distal phalanges.
Fig. 323. - Dissection showing the muscles of the forearm, especially the long flexor muscles of the fingers.
2. The flexor sublimis digitorum arises from the medial (internal) condyle of the humerus, the coronoid process, the intermuscular septa, and the oblique line of the radius and divides into four tendons which split in front of the proximal phalanges to allow the profundus to pass through and then unite again and insert into the sides of the middle phalanges. There are only four instead of five slips, because the thumb has no middle phalanx but only proximal and distal ones (Fig. 323).
3. The palmaris longus arises from the medial (internal) condyle of the humerus and intermuscular septa and inserts into the palmar fascia, which is attached to the base of the proximal phalanges, to the heads of the metacarpal bones, and blends with the capsules of the metacarpophalangeal joints. It is thus seen to be a perforated muscle exactly like the flexor sublimis, which it also resembles in function; its attachment is not so far forward. Traction on it tends to flex the proximal phalanx.
Three separate slips forming the extensor longus pollicis, extensor brevis pollicis, and extensor ossis metacarpi pollicis go to the thumb. The longus inserts into the distal phalanx, the brevis into the proximal, and the ossis into the metacarpal bone of the thumb.
The extensor communis digitorum divides into four slips, one for each finger. The slip to the index is reinforced by an additional one called the extensor indicis proprius muscle. The slip to the little finger is reinforced by the extensor minimi digiti (ext. digiti quinti proprius) muscle. They divide on the dorsum of the proximal phlanges into three parts, the middle one inserts into the base of the middle phalanx, while the two lateral slips insert into the base of the distal phalanx.