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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
tendons of the Abductor pollicis longus and of the Extensores pollicis longus and brevis.
  The Extensor indicis proprius (Extensor indicis) is a narrow, elongated muscle, placed medial to, and parallel with, the preceding. It arises, from the dorsal surface of the body of the ulna below the origin of the Extensor pollicis longus, and from the interosseous membrane. Its tendon passes under the dorsal carpal ligament in the same compartment as that which transmits the tendons of the Extensor digitorum communis, and opposite the head of the second metacarpal bone, joins the ulnar side of the tendon of the Extensor digitorum communis which belongs to the index finger.

Variations.—Doubling; the ulnar part may pass beneath the dorsal carpal ligament with the Extensor digitorum communis; a slip from the tendon may pass to the index finger.

Nerves.—The Brachioradialis is supplied by the fifth and sixth, the Extensores carpi radialis longus and brevis by the sixth and seventh, and the Anconæus by the seventh and eighth cervical nerves, through the radial nerve; the remaining muscles are innervated through the deep radial nerve, the Supinator being supplied by the sixth, and all the other muscles by the seventh cervical.

Actions.—The muscles of the lateral and dorsal aspects of the forearm, which comprise all the Extensor muscles and the Supinator, act upon the forearm, wrist, and hand; they are the direct antagonists of the Pronator and Flexor muscles. The Anconæus assists the Triceps in extending the forearm. The Brachioradialis is a flexor of the elbow-joint, but only acts as such when the movement of flexion has been initiated by the Biceps brachii and Brachialis. The action of the Supinator is suggested by its name; it assists the Biceps in bringing the hand into the supine position. The Extensor carpi radialis longus extends the wrist and abducts the hand. It may also assist in bending the elbow-joint; at all events it serves to fix or steady this articulation. The Extensor carpi radialis brevis extends the wrist, and may also act slightly as an abductor of the hand. The Extensor carpi ulnaris extends the wrist, but when acting alone inclines the hand toward the ulnar side; by its continued action it extends the elbow-joint. The Extensor digitorum communis extends the phalanges, then the wrist, and finally the elbow. It acts principally on the proximal phalanges, the middle and terminal phalanges being extended mainly by the Interossei and Lumbricales. It tends to separate the fingers as it extends them. The Extensor digiti quinti proprius extends the little finger, and by its continued action assists in extending the wrist. It is owing to this muscle that the little finger can be extended or pointed while the others are flexed. The chief action of the Abductor pollicis longus is to carry the thumb laterally from the palm of the hand. By its continued action it helps to extend and abduct the wrist. The Extensor pollicis brevis extends the proximal phalanx, and the Extensor pollicis longus the terminal phalanx of the thumb; by their continued action they help to extend and abduct the wrist. The Extensor indicis proprius extends the index finger, and by its continued action assists in extending the wrist.
 
1F. The Muscles and Fasciæ of the Hand
 
  The muscles of the hand are subdivided into three groups: (1) those of the thumb, which occupy the radial side and produce the thenar eminence; (2) those of the little finger, which occupy the ulnar side and give rise to the hypothenar eminence; (3) those in the middle of the palm and between the metacarpal bones.

Volar Carpal Ligament (ligamentum carpi volare).—The volar carpal ligament is the thickened band of antibrachial fascia which extends from the radius to the ulna over the Flexor tendons as they enter the wrist.

Transverse Carpal Ligament (ligamentum carpi transversum; anterior annular ligament) (Figs. 421, 422).—The transverse carpal ligament is a strong, fibrous band, which arches over the carpus, converting the deep groove on the front of the carpal bones into a tunnel, through which the Flexor tendons of the digits and the median nerve pass. It is attached, medially, to the pisiform and the hamulus of the hamate bone; laterally, to the tuberosity of the navicular, and to the medial part of the volar surface and the ridge of the greater multangular. It is continuous, above, with the volar carpal ligament; and below, with the palmar aponeurosis. It is crossed by the ulnar vessels and nerve, and the cutaneous branches of the median and ulnar nerves. At its lateral end is the tendon of th

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