238 MUSCLES OF THE HAND. 



Relations. By its external surface with the same relations as the exten 

 sor ossis metacarpi. By its deep surfacevfith the ulna, interosseous mem- 

 brane, radius, wrist joint, radial artery, and metacarpal bone of the thumb 

 The muscle is placed between the extensor primi internodii and extensor 

 indicis. 



The EXTENSOR INDICIS arises from the ulna, as high up as the extensor 

 ossis metacarpi pollicis, and from the interosseous membrane. Its tendon 

 passes through a distinct groove in the radius, and is inserted into the 

 aponeurosis formed by the common extensor tendon of the index finger. 



Relations. The same as those of the preceding muscle, with the ex- 

 ception of the hand, where the tendon rests upon the metacarpal bone of 

 the fore finger and second interosseous muscle, and has no relation with 

 the radial artery. 



The tendons of the extensors, as of the flexor muscles of the fore-arm, 

 are provided with synovial bursse as they pass beneath the annular liga- 

 ments : those of the back of the wrist have distinct sheaths, formed by the 

 posterior annular ligament. 



Actions. The anconeus is associated in its action with the triceps ex- 

 tensor cubiti : it assists in extending the fore-arm upon the arm. The 

 supinator longus and brevis effect the supination of the fore-arm, and an- 

 tagonize the two pronators. The extensor carpi radialis longior and bre- 

 vior, and ulnaris, extend the wrist in opposition to the two flexors of the 

 carpus. The extensor communis digitorum restores the fingers to the 

 straight position, after being flexed by the two flexors, sublimis and pro- 

 fundus. The extensor ossis metacarpi, primi internodii, and secundi in- 

 ternodii pollicis, are the especial extensors of the thumb, and serve to 

 balance the actions of the flexor ossis metacarpi, flexor brevis, and flexor 

 longus pollicis. The extensor indicis gives the character of extension to 

 the index finger, and is hence named " indicator," and the extensor 

 minimi digiti supplies that finger with the power of exercising a distinct 

 extension. 



MUSCLES OF THE HAND. 



Radial or Thenar Region. 



Abductor pollicis, Flexor brevis pollicis, 



Flexor ossis metacarpi (opponens), Adductor pollicis. 



Dissection. The hand is best dissected by making an incision along 

 the middle of the palm, from the wrist to the base of the fingers, and 

 crossing it at each extremity by a transverse incisio^, then turning aside 

 the flaps of integument. For exposing the muscles of the radial region, 

 the removal of the integument and fascia on the radial side will be suffi- 

 cient. 



The ABDUCTOR POLLICIS is a small, thin muscle, which arises from the 

 scaphoid bone and annular ligament. It is inserted into the base of the 

 first phalanx of the thumb. 



Relations. By its superficial surface, with the external portion of the 

 palmar fascia. By its deep surface, with the flexor ossis metacarpi. On 

 its inner side it is separated by a narrow cellular interspace from the flexor 

 brevis pollicis. 



This muscle must be divided from its origin and turned upwards, in 

 order to see the next. 



