FLEXOR DIGITORUM PROFUNDUS 401 



c. Third Layer 



(Figs. 372-376) 



The two muscles which constitute this layer may be looked upon as differen- 

 tiated from a single deep flexor muscle. The flexor digitorum profundus is a 

 strong, broad muscle which arises from the upper three-fourths of the volar surface 

 of the ulna and gives rise to tendons which are inserted into the bases of the third 

 row of phalanges of the fingers. The flexor poUicis longus, like\\dse broad and flat, 

 arises from the volar surface of the radius and is inserted into the base of the 

 second phalanx of the thumb. Both muscles are supplied by the median nerve 

 and the flexor profundus is also supplied by the ulnar nerve. 



These muscles correspond to the flexor digitorum longus and the flexor hallu- 

 cis longus of the leg. 



The flexor digitorum profundus (figs. 372-376). — Origin. — (1) Through an aponeurotic 

 septum between it and the flexor carpi ulnaris from the dorsal border of the ulna; (2) directly 

 from the proximal two-thirds of the medial surface and the proximal three-fourths of the volar 

 surface of the ulna and from the adjacent interosseous membrane; and (3) inconstantly, from a 

 small area on the radius below the bicipital tuberosity. 



Structure and insertion. — The fibre-bundles descend nearly vertically and give rise to a 

 common belly which soon divides into four portions, each of which is attached about midway 

 down the forearm in a semipenniform manner to the dorsal surface of a tendon. The attachment 

 of fibre-bundles continues nearly to the wrist. The digital divisions of the muscle vary in the 

 height to which they extend. That belonging to the index finger is usually the one most exten- 



FiG. 373. — Insertions of the Tendons of the Muscles which act on the Finger. 

 (After Toldt, Atlas of Human Anatomy, Rebman, London and New York.) 



Lumbricalis 



Vincula tendinum 



' Tendon of flexor digi- 

 Tendon of flexor digitorum sublimis 



sively isolated, and that to the little finger is the next most so. The tendons pass side by side 

 under the transverse carpal (anterior annular) ligament, and then diverge to the bases of the 

 fingers. At the metacarpo-phalangeal joints, they enter the osteo-fibrous canals described 

 above (p. 387). On the volar surface of the first phalanx each tendon passes through the slit 

 in the sublimis tendon. The tendon then is continued over the second phalanx to the base of the 

 third. Vincula tendinum are described passing to the capsule of the second interphalangeal joint 

 (ligamentum breve) and to the tendon of the flexor sublimis (ligamentum longum). The 

 lumbrical muscles arise from the tendons while they are in the palm. 



Nerve-supply. — The interosseous branch of the median nerve arises usually before the 

 nerve passes through the pronator teres and accompanies the main trunk. This branch as it 

 passes beneath the flexor sublimis gives off a branch (or two) from which several twigs spring. 

 These twigs enter the muscle near the radial border and pass in across the middle third of the 

 constituent fibre-bundles of the fasciculi to the index and middle fingers. The ulnar nerve near 

 the elbow gives rise to a branch which enters the volar surface of the muscle near the junction 

 of the proximal and middle thirds of that portion of the belly, giving tendons to the ring and 

 little fingers. There is some variation in the extent of the innervation by the branches of the 

 ulnar and those of the median nerve. To a greater or less extent through anastomosis their 

 territories overlap. The nerve fibres arise from the seventh and eighth cervical and first 

 thoracic nerves. 



Action. — To flex the terminal phalanx of each finger on the second and the second on 

 the first, while that of the superficial fltxor is to flex the second phalanx on the first. The 

 action of the two flexors on the first phalanx is somewhat more limited. The interosseous 

 muscles, aided by the lumbricals, are the chief flexors of the first row of phalanges. The flexor 

 profundus acts, though not powerfully, as a flexor of the wrist. 



Relations. — The flexor profundus muscle lies beneath the flexor sublimis and the flexor carpi 

 ulnaris muscles, the median nerve, and the ulnar vessels and nerve. Under the muscle lie the 

 ulna, the interosseous membrane, and the pronator quadratus muscle. Under the transverse 

 carpal (anterior annular) ligament the tendons lie beneath those of the flexor sublimis in the 

 same synovial sac. In the palm the tendons with the associated lumbrical muscles lie upon the 

 interosseous muscles, the adductor of the thumb, and the deep palmar arch, and beneath the 

 flexor sublimis tendons. For the relations to the synovial bursse see p. 403. 



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