388 A MANUAL OF ANATOMY 



in doing so, the nerve passes between the small superficial and large 

 deep lamina, of which the muscle is composed. 



2. Extensor Ossis Metacarpi PoUicis — Origin. — (i) The posterior 

 surface of the shaft of the radius for fully 2 inches below the posterior 

 oblique line, where it meets the supinator radii brevis ; (2) the ad- 

 jacent portion of the interosseous membrane ; and (3) the outer 

 part of the posterior surface of the shaft of the ulna for a short 

 distance below the oblique line, which limits the insertion of the 

 anconeus. 



Insertion. — ^The outer side of the base of the metacarpal bone of 

 the thumb. 



The muscle passes downwards and outwards, and its strong tendon 

 is closely accompanied by that of the extensor brevis pollicis. Both 

 of these tendons cross those of the radial extensors of the wrist, and 

 traverse the groove on the outer side of the styloid process of the 

 radius, where they occupy the outermost compartment of the 

 posterior annular ligament. The tendon at its insertion gives a 

 slip to the thenar portion of the palmar fascia, which gives it a 

 double appearance. 



Nerve-supply. — The posterior interosseous nerve. 



Action. — (i) To extend and abduct the metacarpal bone of the 

 thumb, and (2) to abduct the wrist-joint (external lateral flexion). 



3. Extensor Brevis Pollicis (extensor primi internodii pollicis) — 

 Origin. — ^The posterior surface of the shaft of the radius, and the inter- 

 osseous membrane, for a short distance below the preceding muscle. 



Insertion. — The back of the base of the first phalanx of the thumb. 



The muscle closely accompanies the extensor ossis metacarpi 

 pollicis. 



Nerve-supply. — The posterior interosseous nerve. 



Action. — (i) To extend the metacarpo-phalangeal joint of the 

 thumb, and (2) to act as a feeble auxiliary to the extensor ossis 

 metacarpi pollicis. 



4. Extensor Longus Pollicis (extensor secundi internodii pollicis) 

 — Origin. — (i) The outer part of the posterior surface of the shaft of 

 the ulna over about its middle third, commencing immediately below 

 the extensor ossis metacarpi pollicis ; and (2) the adjacent portion 

 of the interosseous membrane. 



Insertion. — ^The back of the base of the distal phalanx of the 

 thumb. 



The muscle is directed downwards and outwards, and its tendon 

 passes beneath the posterior annular ligament, where it occupies the 

 narrow oblique groove on the radius, internal to the radial tubercle. 



Nerve-supply. — ^The posterior interosseous nerve. 



Action. — ^To extend the distal phalanx of the thumb. Thereafter 

 the muscle, which acts strongly, assists in extending the first 

 phalanx and metacarpal bone, the thumb being drawn backwards. 

 It is also an auxiliary extensor of the wrist-joint. 



5. Extensor Indicis — Origin. — (i) The outer part of the posterior 

 surface of the shaft of the ulna, commencing just below the middle 



