EXTENSOR MUSCLES OF THE THUMi3. 



219 



the groove on the outer border of the lower extremity of the radius, 

 and is inserted in the base of the metacarpal bone of the thumb. 



Ikdations. — The upper part of this muscle is concealed by the common 

 extensor, but it becomes superficial beneath, and together with the next muscle 

 crosses the tendons of the radial extensors of the cai-j)us. conceals the insertion 

 of the supinator long-us, and, beneath the extremity of the radius, crosses the 

 radial artery. 



The extensor primi intemodii pollicis muscle lies close to the 

 lower border of the extensor ossis metacarpi, and is much smaller than 

 that muscle ; it arises from the interosseous ligament and tlie radius near 

 the middle of the foreann : its tendon accompanies that of the extensor 

 ossis metacarpi through the same compartment of the annular liga- 

 men , and passes on to be inserted into the proximal end of the first 

 phalanx. 



The extensor secundi internodii poIli.cis muscle, much larger than 

 the extensor primi internodii, which it overlaps, arises immediately below 

 the extensor ossis metacarpi from the shaft of the ulna for two or three 

 inche and from the interosseous ligament at its lower end for about 



Fig. 178.— Superficial JIus- 

 OLES AND Tendons on the 

 Back of the Wrist and 

 Hand (after Bourgery). i 



The posterior annular liga- 

 ment of the wrist is represented. 

 1, extensor ossis metacarpi \.o\- 

 licis ; 1', its insertion ; '2, ex- 

 tensor primi internodii polHcis ; 

 2', its insertion ; 3, 3 , tendon 

 of the extensor secundi inter- 

 nodii poHicis ; 4, extensor com- 

 munis digitorum ; 4', tendon to 

 the middle finger, receiving the 

 insertion of the second and third 

 dorsal interossei ; 4", division of 

 the tendon into three portions, 

 of which the median is inserted 

 into the second phalanx, the two 

 lateral passing on to be inserted 

 into the terminal i)halanx ; the 

 union of the tendons of the 

 third, fourth, and fifth fingers 

 is shown ; 5, extensor minimi 

 digiti ; 5', its junction with the 

 slip of the common extensor ; 

 6, placed on the lower end of 

 the ulna, points to the extensor 

 carpi nlnaris ; 6', insertion of 

 this muscle into the base of the 

 fifth metacarpal bone ; 7, part 

 of the flexor carpi nlnaris ; 8, 

 placed on the os magnum, points 

 to the insertion of the extensor 

 carjii radialis brevior ; 8', placed 



on the base of the second metacarpal bone, points to the insertion of the extensor carpi 

 radialis longior ; 9, tendon of the extensor indicis ; 10, small part of the adductor 

 pollicis, and deep head of the flexor brevis pollicis ; 11, first dorsal interosseous or 

 abductor indicis : in the other three interosseous spaces are seen in succession, from the 

 radial side inwards, the insertion of the first pahnar, second dorsal, third dorsal, second 

 palmar, fourth dorsal, and third palmar interossei muscles ; 12, abductor minimi digiti. 



