[Fig. 87].—Radius and Ulna, Medial or Flexor Side, with M. pronator quadratus and the Areas of Attachment of Other Muscles.

a, M. pronator quadratus; b, tendon of M. biceps; c, conjoined tendon of M. brachialis and M. clavobrachialis; d, insertion of caput longum of M. triceps; e, insertion of short portion of caput mediale of M. triceps; f, origin of M. flexor carpi ulnaris; g, origin of first head of M. flexor profundus digitorum; h, origin of fifth head of M. flexor profundus digitorum; i, intermediate portion of caput mediale of M. triceps.

The second head has origin ([Fig. 82], n) from the distal end of the medial epicondyle of the humerus between the flexor ulnaris and the third and fourth heads of the profundus. It may be very closely attached to these muscles at their origins. Its tendon joins the radial border of the tendon of the first head. From this junction the radial part of the flexor sublimis arises.

The third head ([Fig. 77], u) has origin ([Fig. 82], o) by a strong tendon from the medial epicondyle of the humerus, between the palmaris (p) and flexor radialis (m). Its tendon forms the radial part of the common tendon.

The fourth head has origin by a strong tendon from the medial epicondyle of the humerus just ventrad of the origin of the second head and the flexor ulnaris, to which many of its fibres are attached. Its tendon forms the middle and superficial part of the common tendon.

The fifth or radial head ([Fig. 75], q; [Fig. 77], u′) has origin ([Fig. 87], h) from the middle third of the ventral surface of the shaft of the radius over an area limited by two oblique bony ridges; from the adjacent parts of the interosseous membrane; and from the ventral part of the medial surface of the shaft of the ulna between a point about two centimeters distad of the semilunar notch and the junction of the middle and distal thirds of the shaft. Its very thick tendon forms the middle and deep part of the common tendon.

The common tendon ([Fig. 88], c′) covers the carpus and metacarpus ventrally. It divides into five tendons (c) which pass to the five digits and are inserted into the bases of the terminal phalanges. Those of the first four digits perforate the tendon of the flexor sublimis (b) at the base of the first phalanx of each digit. At the same place each passes through a fibrous pulley-ring attached to the base of the phalanx. Each then passes through a second pulley-ring (2) near the head of the phalanx and is finally inserted into the base of the terminal phalanx.

Relations.—Outer surface with the pronator teres ([Fig. 77], q), flexor carpi radialis ([Fig. 77], r), palmaris longus (s), flexor carpi ulnaris (t), integument, and extensor carpi ulnaris ([Fig. 75], o). Inner surface with the radius, ulna, interosseous membrane and pronator quadratus ([Fig. 87], a).

Action.—Flexor of all the digits.