ANATOMY IN A NUTSHELL. 85 



Origin. — Upper two-thirds of external supracondylar ridge of humerus 

 and external intermuscular septum. (Plate XXIII.) 



Insertion. — Outer side of base of styloid process of raidus. (Plate XXV. 



Action. — Supinates hand. 



Nerve Supply. — Sixth cervical through the musculo-spiral nerve. 



Blood Supply. — Brachial and radial arteries. 



Extensor carpi radialis longior. — Description. — (Plate XXXII.) — 

 This muscle lies partly beneath the Supinator longus, its fibers terminating 

 at the upper third of the forearm in a flat tendon which runs along the 

 outer border of the radius beneath the extensor tendons of the thumb. It 

 then passes through a groove common to it and the Ex'.ensor carpi radialis 

 brevior. immediately behind the styloid process of the radius. 



Origin. — From the lower third of the external supracondylar ridge of the hu- 

 merus and from the external intermuscular septum by a few fibers from the 

 common tendon of origin of the extensor muscles of the forearm. (Plate XXIII.) 



Insertion. — Base of metacarpal of index finger, radial side. (Plate XXVI.) 



Action. — Extends the wrist. 



Xerve Supply. — Sixth and seventh cervical through the musculo-spiral. 



Blood Supply. — Radial artery. 



Extensor carpi radialis brevior. — Description. — (Plates XXXII.) — 

 This muscle is shorter and thicker than the preceding, beneath which it is 

 placed. Its fibers terminate about the middle of the forearm in a flat ten- 

 don which is closely connected with that of the Longior, and accompanies 

 it to the wrist, lying in the same groove on the posterior surface of the radius; 

 it passes beneath the extensor tendons of the thumb, then beneath the annular 

 ligament. 



The tendons of the two preceding muscles pass through the same com- 

 partment of the annular ligament and are lubricated by a single synovial mem- 

 brane, but are separated from each other by a small vertical ridge of bone as 

 they lie in the groove at the back of the radius. 



Origin. — From the external condyle of the humerus by a tendon common 

 to it and the three following muscles: from the external lateral ligament of the 

 elbow-joint, from a strong aponeurosis which covers its surfaces, and from the 

 intermuscular septa. (Plate XXIII.) 



Insertion. — (Plate XXVT.) — Base of metacarpal bone of middle finger 

 on its radial side. 



Action. — It assists the Extensor carpi radialis longior in extending the 

 wrist and may also act slightly as an abductor of the hand. 



Nerve Supply. — Sixth and seventh cervical through the posterior inter- 

 osseous. 



Blood Supply. — Radial artery. 



Extensor communis dijritoruni. — Description. (Plates XXXII.) — This 

 muscle is situated at the back part of the forearm and divides just below the 

 middle into three fleshy masses, from which tendons proceed; these pass to- 

 gether with the Extensor indicis through a separate compartment of the annu- 

 lar ligament. The tendons then diverge, the innermost one dividing into two. 



