Part 17

_Insertion_ by a short tendon into the ridge which forms the ventral limit of the groove on the outer surface of the styloid process of the radius and upon the adjacent ligaments.

_Relations._--Outer surface with the integument, and at the proximal end with the caput laterale of the triceps (Fig. 75, _h_). Inner surface with the brachialis (_i_), the extensor carpi radialis longus (_l_) and brevis, and the extensor communis digitorum (_m_).

_Action._--Supinator of the hand.

=M. extensor carpi radialis longus= (Fig. 75, _l_; Fig. 77, _o_).--This lies along the radial side of the forearm, and connects the humerus and the second metacarpal.

_Origin_ (Fig. 81, _k_) from the middle portion of the lateral supracondyloid ridge between the origin of the brachialis (_j_) and the anconeus. About the middle of the forearm the muscle ends in a slender tendon (Fig. 84, _e_) which passes through the groove on the radial side of the dorsal surface of the distal end of the radius.

_Insertion_ (Fig. 84, _c_).--The tendon passes across the carpus and is inserted onto the dorsal surface of the base of the second metacarpal on the radial side just distad of the groove for the radial artery.

_Relations._--Outer surface at the proximal end with the caput laterale of the triceps (Fig. 75, _h_), farther distad with the integument and the brachioradialis (_k_); the distal tendon with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the brachialis (Fig. 75, _i_) and the integument. Ulnar surface with the extensor carpi radialis brevis (Fig. 77, _p_), and near the proximal end with the extensor communis digitorum (Fig. 75, _m_).

_Action._--Extensor of the hand.

=M. extensor carpi radialis brevis= (Fig. 77, _p_).--A slender muscle from the humerus to the third metacarpal.

_Origin_ (Fig. 81, _l_) from the distal part of the lateral supracondyloid ridge distad of the extensor longus. Near the carpus the muscle ends in a slender tendon which passes through the groove with the extensor longus tendon. The tendon then diverges from the extensor longus and has its

_Insertion_ (Fig. 84, _f_) into the radial side of the dorsal surface of the base of the third metacarpal.

_Relations._--Outer surface with the extensor longus (Fig. 77, _o_), the integument, and distad with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the extensor longus. Ulnar surface with the extensor communis digitorum (Fig. 75, _m_) and the extensor brevis pollicis. Inner surface with the pronator teres (Fig. 77, _q_), supinator (Fig. 85, _b_), and extensor brevis pollicis (Fig. 85, _a_).

_Action._--Extensor of the hand.

=M. extensor digitorum communis= (Fig. 75, _m_).--A slender muscle from the lateral supracondyloid ridge of the humerus to the phalanges of the second, third, fourth, and fifth digits.

_Origin_ (Fig. 81, _m_).--The distal surface of the lateral supracondyloid ridge dorsad of the origin of the extensor carpi radialis brevis.

At the junction of the middle and distal thirds of the radius the muscle passes into a large, flat tendon (Fig. 75, _m′_) which passes through the groove on the middle of the dorsal surface of the distal end of the radius. The groove is converted into a canal by a strong transverse ligament. The tendon divides near the groove into four portions (Fig. 84, _a_) which diverge and pass to the four ulnar digits.

_Insertions._--Each division of the tendon as it passes over the dorsal surface of the first phalanx has its border connected by strong fascia to the dorsal surface of the phalanx. It finally passes through the groove at the distal end of the first phalanx and is inserted mostly into the base of the second phalanx; a portion continues distad to be inserted into the third phalanx.

_Relations._--Outer surface with the integument and at the origin with the caput laterale (Fig. 75, _h_) of the triceps. Radial surface with the extensor longus (_l_) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (_n_). Inner surface with the supinator (Fig. 85, _b_) and the extensor pollicis brevis (Fig. 85, _a_).

_Action._--Extensor of the four ulnar digits.

=M. extensor digitorum lateralis= (Fig. 75, _n_). (This corresponds in position to the extensor digiti quinti proprius of man.)

_Origin_ (Fig. 81, _n_) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis.

The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis (Fig. 75, _m_) and the extensor carpi ulnaris (_o_). Near the carpus it ends in a tendon (_n′_), which divides immediately into three or four (Fig. 84, _b_). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon.

_Insertion_ (Fig. 84, _b_).--The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (_a_) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (_c_) and may also give a branch to the radial side of the base of the first phalanx of the third digit.

_Relations._--Outer surface with the integument. Radial border with the extensor communis digitorum (_m_); ulnar border with the extensor carpi ulnaris (_o_). Inner surface with the supinator (Fig. 85, _b_) and the extensor brevis pollicis (Fig. 85, _a_); distad with the extensor indicis (Fig. 85, _c_).

_Action._--Extensor of the four ulnar digits.

=M. extensor carpi ulnaris= (Fig. 75, _o_).

_Origin._--(1) By a short, broad tendon from the distal portion of the lateral epicondyle of the humerus (Fig. 81, _o_) distad of the origin of the extensor lateralis, and (2) by a smaller tendon from the ulna at the dorsal tip of the semilunar notch (Fig. 86, _e_).

The muscle passes along the ulnar side of the forearm dorsad of the extensor digitorum lateralis (_n_). It ends near the junction of the middle and distal thirds of the forearm in a large, flat tendon (_o′_) which is closely adherent to the ligaments of the wrist and which passes over the distal end of the ulna to its

_Insertion_ into the tubercle on the ulnar side of the base of the fifth metacarpal (Fig. 84, _g_).

_Relations._--Outer surface with the integument. Radial (or ventral) border with the extensor digitorum lateralis (Fig. 75, _n_); ulnar (or dorsal) border with the flexor profundus digitorum (_q_). Inner surface with the radius, the extensor indicis (Fig. 85, _c_), and the extensor pollicis brevis (Fig. 85, _a_).

_Action._--Indicated by its name.

=M. extensor indicis (proprius)= (Fig. 85, _c_; Fig. 75, _p_) (includes extensor pollicis longus also).

_Origin_ (Fig. 86, _f_) by short, fleshy fibres from the lateral surface of the ulna between the semilunar notch and the junction of the third and fourth (distal) fourths.

The muscle-fibres pass obliquely distad and toward the radial side and join the tendon (Fig. 85, _c′_), which runs nearly the whole length of the radial surface.

_Insertion_ (Fig. 84, _c_).--The tendon passes through the groove on the ulnar side of the dorsal surface of the base of the radius and divides into two. Both of these usually go to the base of the second phalanx of the second digit, but one may pass to the pollex, or there may be three divisions going to the first three digits.

_Relations._--Outer surface with the integument, the extensor carpi ulnaris (Fig. 75, _o_), and the extensor digitorum lateralis (Fig. 75, _n_). Dorsal border with the flexor profundus digitorum (Fig. 75, _q_). Inner surface with the extensor brevis pollicis (Fig. 85, _a_).

_Action_ as indicated by the name.

=M. supinator= (Fig. 85, _b_).--A flat muscle wrapped spirally about the proximal end of the radius.

_Origin_ by a short, strong tendon from the lateral side of the annular ligament of the radius and by tendinous fibres from the radial collateral ligament passing from the humerus to the radius. From the tendon of origin the fibres diverge, passing toward the radial side and distad.

_Insertion_ (Fig. 86, _g_).--The dorsal and part of the ventral surface of the proximal two-fifths of the radius to within five millimeters of the articular face of the head.

_Relations._--Outer surface with the extensor carpi radialis brevis, extensor communis digitorum (Fig. 75, _m_), and extensor lateralis digitorum (Fig. 75, _n_). Radial border with the pronator teres (Fig. 77, _q_). Inner surface with the radius.

_Action._--Supinator of the hand.

=M. extensor brevis pollicis= or extensor ossis metacarpi pollicis (Fig. 85, _a_) (includes M. abductor longus pollicis of man).

_Origin_ (Fig. 86, _h_) by fleshy fibres from the ventral half of the lateral surface of the shaft of the ulna from the semilunar notch to the styloid process; from the ulnar half of the dorsal surface of the radius from the bicipital tubercle to a short distance distad of the middle of the bone; and from the interosseous membrane between these two areas. The fibres converge to form a strong, flat tendon (Fig. 84, _d_) which passes obliquely over the tendons of the extensor carpi radialis longus (_e_) and brevis (_f_) to its

_Insertion_ into the radial side of the base of the first metacarpal. The radial sesamoid bone of the wrist is imbedded in the tendon at its insertion.

_Relations._--Outer surface with the extensor carpi radialis brevis, the extensor communis digitorum (Fig. 75, _m_), extensor lateralis digitorum (Fig. 75, _n_), and extensor indicis (Fig. 85, _c_). Inner surface with the radius and ulna, and distad with the tendons of the two extensores carpi radiales (Fig. 84, _e_ and _f_).

_Action._--Extends and abducts the pollex.

B. MUSCLES ON THE RADIAL AND VENTRAL SIDE OF THE FOREARM (flexors and pronators).

=M. pronator teres= (Fig. 77, _q_).

_Origin_ (Fig. 82, _l_) by a short strong tendon from the extremity of the medial epicondyle of the humerus.

_Insertion_ (Fig. 86, _i_) by fleshy fibres and short tendinous fibres along the medial border of the radius, at its middle. The radial edge of the muscle is continuous with the strong deep layer of the antibrachial fascia.

_Relations._--Outer surface with the superficial fascia. Radial border with the tendon of the biceps (Fig. 79, _l′_), with the extensor carpi radialis brevis (Fig. 77, _p_), and the supinator (Fig. 85, _b_). Ulnar border with the flexor carpi radialis (Fig. 77, _r_) and part of the flexor profundus digitorum (Fig. 77, _u_).

_Action._--Pronates the hand by rotating the radius.

=M. flexor carpi radialis= (Fig. 77, _r_).--A slender fusiform muscle.

_Origin_ (Fig. 82, _m_) from the tip of the medial epicondyle of the humerus.

_Insertion._--The slender tendon passes through a deep groove between the os magnum and the first metacarpal. The groove is converted into a canal by the overlying tendons and muscles. The tendon is finally inserted into the bases of the second and third metacarpals.

_Relations._--Outer surface with the pronator teres (Fig. 77, _q_), the superficial fascia, and the third head of the flexor profundus (_u_). Radial border with the pronator teres (_q_). Ulnar border with the third and fourth heads of the flexor profundus. Inner surface with the ulna proximad and the fifth head of the flexor profundus (_u′_) distad.

_Action_ indicated by the name.

=M. palmaris longus= (Fig. 77, _s_).--A flat fusiform muscle beneath the fascia on the medial border of the forearm.

_Origin_ (Fig. 82, _p_) by a short flat tendon from the distal part of the medial surface of the medial epicondyle of the humerus.

About one centimeter from the wrist the muscle ends in a flat tendon which passes through the transverse ligament and divides on the hand into four or five (or sometimes only three) tendons (Fig. 77, _s′_) which diverge to the insertions. The ulnar portion of the flexor sublimis digitorum (Fig. 77, _x_) takes origin from the common tendon before its division.

_Insertion._--Each tendon except the first gives off a branch which spreads out in the trilobed pad in the palm. The middle two of these may be traced to the integument covering the pad. The tendon is then inserted (Fig. 88, _a_) near the base of the first phalanx onto the outer surface of the perforated portion of the flexor sublimis tendon (_b_, _b′_) at its side, except that to the thumb, which divides near its distal end into two which are inserted into the sesamoid bones at the base of the first phalanx. The tendons of insertion are closely united to the fibrous pulley-ring at the base of the first phalanx, and each may send a slip to the base of the phalanx at either side of the ring.

_Relations._--Outer surface with the superficial fascia and one head of the flexor sublimis (Fig. 77, _x_). Radial border with the flexor carpi radialis. Ulnar border with the flexor carpi ulnaris and a part of the flexor profundus digitorum. Inner surface with the flexor carpi radialis (_r_), the flexor carpi ulnaris (_t_), the flexor profundus digitorum (_u_), and the radial head of the flexor sublimis digitorum.

_Action._--Flexor of the first phalanx of each of the digits.

=M. flexor carpi ulnaris= (Fig. 77, _t_, _t′_).

_Origin._--There are two heads. The first or humeral head (_t′_) takes origin in common with the second part of the flexor profundus from the median surface of the distal end of the humerus just distad of the medial epicondyle (Fig. 82, _q_). The second or ulnar head (_t_) takes origin by fleshy fibres from the lateral surface of the olecranon and from the dorsal border of the ulna from the olecranon to a point distad of the semilunar notch (Fig. 87, _f_). The two heads join proximad of the middle of the forearm. The muscle passes obliquely across the ventral surface of the forearm to its

_Insertion_ by fleshy and tendinous fibres into the proximal surface of the pisiform bone.

_Relations._--Outer surface with the integument and the palmaris longus (Fig. 77, _s_). Inner surface and ulnar border with the flexor profundus digitorum.

_Action_ indicated by the name.

=M. flexor sublimis digitorum= (or perforatus).--This muscle is in two parts, which are given a common name only because of the similar structure and insertions of their tendons.

The =ulnar part= (Fig. 77, _x_) is a conical muscle taking origin from the outer surface of the tendon and muscle of the palmaris longus and from the adjacent ligament. It gives rise to two or three tendons (Fig. 88, _b_) which arise from distinct slips of the muscle; these pass to the ulnar two or three digits. That passing to the fourth or fifth digit receives an accessory slip from a small mass of fibres attached to the transverse ligament on the radial side of the palmaris tendon.

The =radial part= is a small flat triangular muscle which takes origin from the outer surface of the tendon formed by the junction of the tendons of the first and second parts of the flexor profundus. It divides into two slips, each giving rise to a tendon. These tendons (Fig. 88, _b′_) pass to the second and third digits. That to the third digit may divide into two, one of which goes to the fourth digit.

Each of the four tendons (Fig. 88, _b_ and _b′_) is perforated by a tendon of the flexor profundus (Fig. 88, _c_) as it passes through the fibrous pulley-ring at the base of the first phalanx. It then continues beneath the flexor profundus tendon through the second pulley-ring (2) and is inserted into the base of the second phalanx.

_Relations._--Outer surface of the ulnar part with the integument; inner surface with the palmaris longus (Fig. 77, _s_). Outer surface of the radial part with the palmaris longus; inner surface with the flexor profundus.

_Action._--Flexor of the second phalanx of digits 2-5.

=M. flexor profundus digitorum= (or perforans).--This arises by five heads, the tendons of which join one another at the wrist to make the strongest and deepest of the flexor tendons.

The _first_ or _ulnar head_ has _origin_ (Fig. 87, _g_) from the dorsal half of the medial (radial) surface of the ulna from the proximal lip of the semilunar notch to within a centimeter of the styloid process. Its fibres converge to a large flat tendon which forms the lateral (ulnar) and superficial part of the common tendon.

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.

=M. pronator quadratus= (Fig. 87, _a_).--A thick quadrangular muscle whose fibres run obliquely between the distal ends of the ulna and radius.

_Origin_ by fleshy fibres from about the distal half of the ventral (flexor) surface or border of the ulna and from the interosseous membrane adjacent to the area. The fibres pass obliquely distad toward the radial side to their

_Insertion_ by fleshy fibres into the ventral (flexor) surface of the radius distad of the area of origin of the fifth head of the profundus.

_Relations._--Outer surface with the flexor profundus. Inner surface with the radius, ulna, and interosseous membrane.

_Action_ as indicated by the name.

4. =Muscles of the Hand.=

A. BETWEEN THE TENDONS.--=Mm. lumbricales= (Fig. 88, _f_, _f′_).--Four small muscles in the palm of the hand.

_Origin_ by fleshy fibres from the outer (palmar) surface of the tendon common to the ulnar four parts of the profundus (_c′_).

_Insertion_ (_f′_).--The four slips are flat at their origin. Each becomes cylindrical and curves about the base of one of the four ulnar digits and is inserted into the radial side of the base of the first phalanx close to its ventral border.

_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_).