ULNO-VOLAR DIVISION 395 



Nerve-supply. — By a twig from the deep radial (posterior interosseous) nerve. This twig 

 enters the proximal third of the radial border of the muscle. It frequently arises from a branch 

 to the extensor pollicis longus. The nerve fibres come from the sixth, seventh, eighth cervical 

 nerves. 



Action. — To extend the first phalanx on the metacarpal. Like the common extensor 

 it has a limited action on the two terminal phalanges. It also adducts the index finger and is a 

 weak supinator of the forearm. 



Relations. — It is covered by the superficial extensor group. 



Variations. — These are frequent. It may be absent. There may be two heads, or the 

 muscle may be completely doubled. It may receive an accessory sUp from the ulna or the 

 carpus. The tendon may give accessory slips to the middle finger, the ring finger, or the thumb. 

 The accessory tendon to the middle finger is the most frequent. The tendon to the index 

 finger may be inserted on the metacarpus. 



Abnormal Muscles of the Back of the Wrist and Hand 



The extensor medii digiti is a small muscle which arises from the ulna beneath the extensor 

 of the index finger, with which it is more or less fused. It sends a tendon to the extensor 

 aponeurosis of the middle finger or shps both to this finger and the index finger. It is present 

 in about 10 per cent, of bodies (Le Double). 



The extensor digiti annularis is a muscle similar to the extensor medii digiti, but much 

 rarer . 



The extensor digitorum brevis, which resembles the muscle of corresponding name on the 

 dorsum of the foot, may have from one to four fascicuH, but most frequently one. The most 

 common fasciculus is one which sends a tendon to the extensor tendon of the index finger. One 

 for the middle finger is nearly as frequent. Others are rare. A fasciculus for the thumb has not 

 been reported. (Le Double.) The fasciculi usually arise from the bones of the ulnar half of 

 the carpus — lunatum (semilunar), triquetrum (cuneiform), hamatum (unciform), andcapitatum 

 (magnum), and from the dorsal ligaments uniting these bones. The tendons are inserted either 

 into the corresponding extensor tendons or into the metacarpals. The muscle is found in about 

 10 per cent, of bodies (Wood). 



BURS.E 



B. m. extensoris carpi radialis brevis. — Between the tendon and the base of the third 

 metacarpal. 



B. m. abductoris pollicis longi. — Between the tendons of the long and short radial extensors 

 and the tendons of the abductor pollicis longus and extensor pollicis brevis. Another bursa 

 lies beneath the tendon of insertion of the abductor. 



B. intermetacarpo-phalangeae. — Between the lateral surfaces of the heads of the meta- 

 carpal bones of neighbouring fingers dorsal to the transverse capitular ligarnent. 



B. tendinum m. extensoris digitorum communis. — Small bursse are sometimes found beneath 

 the tendons to the index and httle fingers near where they begin to diverge from the common 

 tendon. 



B. m. extensoris carpi ulnaris. — A smaU bursa may be found under the tendon of origin 

 of this muscle. 



B, m. supinatoris. — Between the supinator and the tendon of the extensor muscles. 



B. m. extensoris pollicis longi. — Between the tendon and the first metacarpal. 



Synovial Tendon-sheaths 



Vagina tendinum mm. extensorum carpi radialium. — Synovial sheaths cover the tendons 

 of the two radial carpal extensors as they pass beneath the dorsal carpal (posterior annular) 

 ligament. In the adult these sheaths usually are more or less fused and communicate with the 

 sheath of the extensor pollicis longus where this crosses them. 



Vagina tendinum mm. extensoris digitorum communis et extensoris indicis.— A synovial 

 sheath surrounds the tendons of these muscles as they pass beneath the dorsal carpal (posterior 

 annular) ligament- This sheath extends for some distance on the tendons as they diverge. 



Vagina tendinis m. extensoris digiti quinti. — A synovial sheath extends on the tendon of 

 this muscle from above the dorsal carpal (posterior annular) ligament to the base of the meta- 

 carpal. 



Vagina tendinis m. extensoris carpi ulnaris. — ^This sheath commences above the carpal 

 (posterior annular) ligament and extends to the insertion of the tendon. 



Vagina tendinum mm. abductoris pollicis longi et extensoris pollicis brevis. — The sheaths 

 which surround these two tendons beneath the dorsal carpal (posterior annular) ligament 

 usually communicate freely. 



Vagina tendinis m. extensoiis pollicis longi. — A long synovial sheath surrounds this 

 tendon. Where it crosses the tendons of the radial extensors, a communication is found with the 

 sheath of the latter. 



2. Ulno-Volar Division 



The muscles on the volar side of the forearm lie in four layers. 



a. First Layer 

 (Fig. 370) 



Of the four muscles of associated ulnar epicondylar origin which constitute 

 this layer the pronator teres is a strong, band-like muscle which is inserted into 



