DEEP MUSCLES. 87 



Beneath this extensor are some of the muscles of the deep layer, relations ; 

 with part of the ulna. On the outer side is the extensor of the 

 littl^ finger, with the posterior interosseous vessels. 



Ai'tion. The ulnar extensor straightens the wrist, and inclines the and use. 

 hand towards the ulnar side : it can then extend the elbow-joint. 



Tlic AXCONEUS (G) is a small triangular muscle near the elbow. Anconeus 

 It art.*'* from the outer condyle of the humerus by a tendon distinct origin; 

 from, and on the ulnar side of the common tendon of the foregoing 

 muscles. From this origin the fibres diverge to their insertion into insertion ; 

 the outer side of the olecranon, and into the impression on the upper 

 third of the posterior surface of the ulna (fig. 36). 



The upper fibres are nearly transverse, and are contiguous to the touches the 

 lowest of the triceps muscle. Beneath the anconeus lie the supinator trice P s : 

 brevis muscle, and the interosseous recurrent vessels. 



Action. It assists the triceps in extending the elbow. use. 



Dissection (fig. 37). For the display of the deep muscles of Dissection 

 the back of the forearm, and of the posterior interosseous vessels and 

 nerve, three of the superficial muscles, viz., extensor communis 

 digitorum, extensor minimi digiti, and extensor carpi ulnaris, are to 

 be divided above and turned aside ; and the small branches of the 

 nerve and artery entering these muscles may be cut. 



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

 and from the ramifications of the artery and nerve ; and a slender and interos. 

 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 deep muscles should be carefully separated, since the outer 

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



DEEP LAYER of MUSCLES (fig. 37). In this layer there are five Five 

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

 muscles of the thumb and index finger. The highest muscle, partly la y er - 

 surrounding the upper third 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, fig. 37, also Extensor 



fig. 36) is the largest and highest of the extensor muscles of nfetacarpi 



the thumb, and is sometimes united with the supinator brevis. It pollicis: 



tt.r'i* x from the posterior surface of the radius in its middle third, origin; 



below the supinator brevis, from a special narrow impression on 



the ulna, occupying the upper third of the outer division 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 insertion; 



base of the metacarpal bone of the thumb, and by a slip into 



the trapezium : another slip is frequently continued to the abductor 



pollicis. 



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

 the fingers; but it becomes superficial in the lower third of the 



