486 THE MUSCLES AND FASCIA 



preceding. It arises from the posterior surface of the shaft of the ulna, below 

 the origin of the Extensor longus pollicis and from the interosseous membrane. 

 Its tendon passes with the Extensor communis digitorum through the same canal 

 in the annular ligament, and subsequently joins the tendon of the Extensor 

 communis which belongs to the index finger, opposite the lower end of the corre- 

 sponding metacarpal bone, lying to the ulnar side of the tendon from the common 

 Extensor. 



Nerves. -The Brachioradialis is supplied by the fifth and sixth, the Extensor carpi radialis 

 longior by the sixth and seventh, and the Anconeus by the seventh and eighth cervical nerves, 

 all through the musculospiral nerve; the remaining muscles of the radial and posterior brachial 

 region are supplied through the posterior interosseous nerve, the Supinator [brevis] being sup- 

 plied by the sixth cervical, the Extensor carpi radialis brevior by the sixth and seventh cervicals, 

 and all the other muscles by the seventh cervical. 



Actions. The muscles of the radial and posterior brachial regions, which comprise all the 

 Extensor and Supinator muscles, act upon the forearm, wrist, and hand; they are the direct 

 antagonists of the Pronator and Flexor muscles. The Anconeus assists the Triceps in extending 

 the forearm. The chief action of the Brachioradialis is that of a Flexor of the elbow-joint, but 

 in addition to this it may act both as a Supinator or a Pronator; that is to say, if the forearm is 

 forcibly pronated it will act as a Supinator, and bring the bones into a position midway between 

 supination and pronation ; and, vice versa, if the arm is forcibly supinated, it will act as a pro- 

 nator, and bring the bones into the same position, midway between supination and pronation. 

 The action of the muscle is therefore to throw the forearm and hand into the position they 

 naturally occupy when placed across the chest. The Supinator f brevis] isatruesupinator; that 

 is to say, when the radius has been carried across the ulna in pronation and the back of the 

 hand is directed forward, this muscle carries the radius back again to its normal position on the 

 outer side of the ulna, and the palm of the hand is again directed forward. The Extensor carpi 

 radialis longior 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 brevior assists the Extensor carpi radialis longior in extending the wrist, and may also act 

 slightly as an abductor of the hand. The Extensor carpi ulnaris helps to extend the hand, but 

 when acting alone inclines it toward the ulnar side ; by its continued action it extends the elbow- 

 joint. The Extensor communis digitorum extends the phalanges, then the wrist, and finally 

 the elbow. It acts principally on the proximal phalanges, the middle and terminal phalanges 

 being extended by the Interossei and Lumbricales. It has also a tendency to separate the 

 fingers as it extends them. The Extensor minimi digiti extends similarly the little finger, and 

 by its continued action it 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 Extensor 

 ossis metacarpi pollicis is to carry the thumb outward and backward from the palm of the hand, 

 and hence it has been called the Abductor pollicis longus. By its continued action it helps to 

 extend and abduct the wrist. The Extensor brevis pollicis extends the proximal phalanx of the 

 thumb. By its continued action it helps to extend and abduct the wrist. The Extensor longus 

 pollicis extends the terminal phalanx of the thumb. By its continued action it helps to extend 

 and abduct the wrist. The Extensor indicis extends the index finger, and by its continued action 

 assists in extending the wrist. It is owing to this muscle that the index finger can be extended 

 or pointed while the others are flexed. 



Applied Anatomy. The tendons of the Extensor muscles of the thumb are liable to become 

 strained and their sheaths inflamed after excessive exercise, producing a sausage-shaped swell- 

 ing along the course of the tendon, and giving a peculiar creaking sensation to the finger when 

 the muscle acts; it is known as tenosynovitis. In piano-players the slips which join the tendons 

 of the Extensor communis digitorum may limit freedom of motion in individual fingers. Their 

 complete extension, without operative interference, can only be brought about by long-continued 

 exertion in practice, when elongation of certain accessory, but restricting, tendons is made by 

 nutritive growth. If there is much limitation, division of the hindering slips is advisable. 1 Par- 

 alysis of the Extensor muscles of the hand is common in lead poisoning, and causes the well- 

 known " ivrixt drop." Curiously enough, the Brachioradialis and Extensor ossei metacarpi 

 pollicis muscles escape. The paralysis is due to a neuritis of the musculospiral nerve. 



i Prof. William S. Forbes, in the Philadelphia Medical Journal, January 15, 1898. 



