DEEP EXTENSOR MUSCLES. 287 



Dissection (fig. 88). For the display of the deep muscles at the back of 

 the forearm, and the posterior interosseous artery and nerve, three of the 

 superficial muscles, viz., extensor communis, extensor minimi digiti, and 

 extensor carpi ulnaris, are to be detached from their origin and turned 

 aside : in this proceeding the small branches of the nerve and artery enter- 

 ing the muscles may be divided. 



The loose tissue and fat are then to be removed from the muscles, and 

 the ramifications of the artery and nerve ; and a slender part of the nerve, 

 which sinks beneath the extensor of the second phalanx of the thumb 

 about the middle of the forearm, should be traced beyond the wrist. 



The separation of the muscles should be made carefully, because the 

 highest two of the thumb are not always very distinct from each other. 



DEEP LAYER OF MUSCLES (fig. 88). In this layer there are five small 

 muscles, viz., one supinator of the forearm, and four special extensor mus- 

 cles of the thumb and forefinger. The highest muscle, surrounding partly 

 the upper end of the radius, is the supinator brevis, D. Below this are the 

 three muscles of the thumb in the following order : the extensor of the 

 metacarpal bone, E, the extensor of the first, F, and that of the second 

 phalanx, G. On the ulna the indicator muscle, H, is placed. 



The EXTENSOR ossis METACARPi POLLICIS, E, is the largest and highest 

 of the extensor muscles of the thumb, and is sometimes united with the 

 supinator brevis. It arises from the posterior surface of the radius for 

 three inches below the supinator brevis ; from the ulna for the same dis- 

 tance by a narrow special impression on the upper and outer part of the 

 posterior surface ; and from the intervening interosseous membrane. The 

 tendon is directed outwards over the radial extensors of the wrist, and 

 through the annular ligament, to be inserted into the base of the metacarpal 

 bone of the thumb, and by a slip into the os trapezium. 



The muscle is concealed at first by the common extensor of the fingers ; 

 but it becomes cutaneous between the last muscle and the extensors of the 

 wrist, about two inches above the end of the radius (fig. 87). Opposite 

 the carpus the radial artery winds backwards beneath its tendon. Between 

 the contiguous borders of this muscle and the supinator brevis the posterior 

 interosseous artery (a) appears. 



Action. By this muscle the thumb is moved backwards from the palm 

 of the hand, and the wrist is extended on the radial side. 



The EXTENSOR PRIMI iNTERNODii POLLICIS, F, is the smallest muscle 

 of the deep layer, and its tendon accompanies that of the preceding ex- 

 tensor. Its origin, about one inch in width, is from the radius and the 

 interosseous membrane, close below the attachment of the preceding muscle. 

 The tendon passes through the same space in the annular ligament as the 

 extensor of the metacarpal bone, and is inserted into the metacarpal end 

 of the first phalanx of the thumb. With respect to surrounding parts this 

 muscle has the same connections as the preceding. 



Action. It extends first the nearest phalanx, and then the wrist, like 

 its companion. 



The EXTENSOR SECUNDI INTERNODII POLLICIS, G, arises from the ulna 

 for four inches below the anconeus, along the ulnar side of the extensor of 

 the metacarpal bone ; and from the interosseous membrane, below, for one 

 inch. Its tendon passing through a sheath in the annular ligament, dis- 

 tinct from that of the other two extensor muscles, is directed along the 

 dorsum of the thumb to be fixed to the base of the last phalanx. 



It is covered by the same muscles as the other extensors of the thumb, 



