30 



THE MUSCLES OF THE UPPER LIMB. 



extensors of the metacarpal bone and first phalanx of the thumb, a bursa being interposed. 

 and below the ligament by the tendon of the long extensor of the thumb. A small bursa is 

 placed under each tendon close to its insertion, and occasionally there is another between the 

 origin of the shorter muscle and the supinator brevis. The synovial sheath surrounding the 

 tendons beneath the annular ligament communicates with that of the extensor longus pollicis 

 at the spot where the latter tendon crosses. 



Varieties. The two foregoing muscles are subject to a similar variation in being, one or 

 other of them, split up into two or sometimes three tendons previous to insertion, the tendons 



Fig. 238. SUPERFICIAL MUSCLES OF THE FOREARM AND 

 HAND, SEEN FROM BEHIND. (Allen Thomson. ) i 



d, olecranon ; e, external condyle ; /, lower end of ulna ; 8", 

 tendon of triceps : 9, anconeus ; 10, part of brachialis anticus ; 

 11, supinator longus; 12, extensor carpi radialis longior; 13, 



brevior ; 14, extensor communis digitorum ; 15, extensor 



carpi ulnaris ; 15', its insertion into the fifth metacarpal bone ; 



+ , between 14 and 15, extensor minimi digiti ; 16, origin of the 

 flexor carpi ulnaris by an aponeurosis from the back of the ulna ; 

 17, extensor ossis metacarpi pollicis ; 17', its insertion into the 

 first metacarpal bone ; 18, extensor brevis pollicis ; 18', its in- 

 sertion into the first phalanx ; 4- and +, posterior annular liga- 

 ment ; at +, the tendons of the long and short radial extensors ; 

 at +, the tendon of the extensor minimi digiti; 19, tendon of 

 extensor longus pollicis ; 20, is placed on the proximal end of the 

 second metacarpal bone, close to the insertion of the radial ex- 

 tensors of the carpus : in the hand, the dorsal interosseous muscles- 

 are shown, and on the middle finger the insertion of the extensor 

 tendon. 



of either muscle being inserted into both the second and third 

 metacarpal bones : occasionally a slip passes also to the fourth 

 metacarpal bone. The two muscles have been seen united, so 

 that a single fleshy belly gives off two tendons. Cross slip* 

 from one muscle to the other are of frequent occurrence, 

 and may be regarded as imperfect forms of the following 

 muscle. 



The extensor carpi radialis intermcdius is rare as a distinct 

 muscle arising independently from the humerus, but is not 

 unf requent as a slip derived from one or both of the normal 

 radial extensors : it is inserted below into the second or third 

 metacarpal, or into both of these bones. 



The extensor carpi radialis acccssorius is an additional 

 muscle sometimes met with, arising from the humerus with 

 or below the extensor carpi radialis longior, and inserted most 

 frequently into the metacarpal bone of the thumb, but some- 

 times into the abductor pollicis, first dorsal interosseous 

 muscle, or other part. It is represented at times by a slip 

 from the tendon of the extensor longior. 



The extensor communis digitorum muscle 

 arises from the outer condyle by the common tendon, 

 from the fascia of the forearm, and from the septa 

 between it and the adjoining muscles. The fleshy 

 mass forms three bellies, the innermost of which becomes divided again ibelow, 

 and from each part a tendon proceeds. The four tendons pass under the posterior 

 annular ligament, lying together with the extensor indicis in the broad inner- 

 most groove on the back of the radius, and descend on the back of the hand to 

 the fingers. The first and second tendons pass respectively to the index and 

 middle fingers, and are usually connected by a weak band of transverse fibres ; the 

 first is accompanied by the tendon of the extensor indicis, the two becoming 

 united opposite the metacarpo-phalangeal articulation. The third tendon runs- 

 mainly to the ring finger, but sends a slip, to join the tendon of the middle finger. 



