THE UPPER LIMB 383 



Muscles of the Outer Side of the Forearm. — ^These are three 

 in number, namel^^ the brachio-radialis, extensor carpi radialis 

 longior, and extensor carpi radialis brevior. 



1. Brachio-radialis (supinator radii longus) — Origin. — (i) The 

 upper two-thirds of the external supracondylar ridge of the humerus, 

 and (2) the front of the external intermuscular septum over a 

 corresponding extent. 



Insertion. — The outer side of the radius close above the base of 

 the styloid process. 



Nerve-supply. — The musculo-spiral nerve. 



Action. — Though called a supinator, the muscle is a flexor of the 

 forearm, acting most directly when the limb is in a state of semi- 

 pronation. An important use of the muscle is to maintain the 

 forearm in the flexed position, as in holding a book. When the 

 forearm is fully pronated the muscle is a feeble supinator, but only 

 to the extent of semisupination. When the forearm is fully supi- 

 nated it produces sem.ipronation. 



The muscle presents a fleshy belly to near the middle of the fore- 

 arm. Thereafter it is replaced by a flat tendon, which first appears 

 on the deep surface of the muscle. Superiorly the surfaces are 

 directed outwards and inwards, but afterwards they look forwards 

 and backwards. 



2. Extensor Carpi Radialis Longior — Origin. — (i) The lower 

 third of the external supracondylar ridge of the humerus, and 

 (2) the front of the external intermuscular septimi for a correspond- 

 ing extent. 



Insertion. — The back of the base of the second metacarpal bone 

 over its outer half, a small bursa lying beneath the tendon. 



Nerve-supply.— The musculo-spiral nerve. 



Action. — (i) To extend the wrist- joint, and (2) to assist in ex- 

 ternal lateral flexion (abduction) of that joint. 



The muscle presents a fleshy belly to near the middle of the fore- 

 arm, and thereafter a tendon which descends at first upon, and 

 then on the outer side of, the tendon of the extensor carpi radialis 

 brevior. These two tendons pass beneath the posterior annular 

 ligament, and occupy the outermost groove on the back of the 

 lower end of the radius. The surfaces of the muscle are at first 

 directed outwards and inwards, and then forwards and backwards. 



3. Extensor Carpi Radialis Brevior — Origin. — (i) The external 

 epicondyle of the humerus by the common tendon ; (2) the external 

 lateral ligament of the elbow- joint ; and (3) the intermuscular septa 

 between it and adjacent muscles. 



Insertion. — ^The back of the base of the third metacarpal bone 

 over its outer half, and often by a small slip into the back of the 

 base of the second metacarpal over its inner part. A small bursa is 

 placed beneath the tendon. 



Nerve-supply. — The posterior interosseous nerve. 



Action. — (i) To extend the wrist-joint, and (2) to assist in 

 external lateral flexion (abduction) of that joint. 



