220 MUSCLES OF THE UPPER LIMB. 



an incn. Its fibres end in a tendon, wliieh occupies the narrow oblique 

 groove in the middle of the posterior surftice of the carpal end of the 

 radius, and is bound down in a separate compartment of the annular liga- 

 ment ; it is inserted into the base of the terminal phalanx of the thumb. 



Hdations. — Tlie groove which, lodges the radial extensors of the carpus inter- 

 venes between tliose which lodge the tendon of tliis muscle and the tendons of 

 the other extensors of the thumb. 



Vdr'u'tics. — The extensor muscles of the thumb are subject to considerable 

 variations, and if all the three muscles be included they seem to occur as often 

 as in one out of every six subjects dissected. Tlie most common occur in the 

 extensor ossis metacari^i, and consist in more or less cleaA^age of the muscle or its 

 tendon into separate parts. The insertion of the distinct tendons takes place 

 either doubly into the first metacaq^al bone, or m part into the trapezium, or 

 into the abductor or opponens poUicis muscles. The extensor primi internodii is 

 sometimes absent, being, as it were, fused with the extensor ossis metacarjii, in 

 other cases it is more or less united with the extensor secundi internodii poUicis. 

 A frequent variety, representing a muscle nonnally existing in the dog and many 

 carnivora, is formed by the interposition of an additional extensor between the 

 indicator and the extensor secundi internodii poUicis, with a double tendon and 

 insertion into both fingers. 



The extensor indicis or indicator muscle arises from the lower end 

 of the posterior surface of the ulna for three or four inches. The 

 tendon passes with the common extensor through a compartment of the 

 annular ligament, comes in contact with the tendon from that muscle 

 destined for the index-finger, and unites with it to form the expansion 

 already described. 



Fig. 179. — Transverse Section op the Right 

 Hand between the Carpus and Meta- 

 carpus. (A. T.) }i 



a, h, c, d, c, articular surfaces of the trape- 

 zium, trapezoid, and unciform bones ; a', palmar 

 ridge of the trapezium ; c', unciform process ; 

 between «' and c, the cut edge of the annular 

 lig'.tnieut, a process t(^vards the trapezium at 

 11, liy -wliich the tendon of the flexor carpi 

 radialis is eaclosed in the groove of the trajje- 

 ziuni ; 1, tendon of the extensor ossis metacarpi 

 Ijollicis ; 2, extensor ^irimi internodii ; 3, ex- 

 tensor secundi internodii ; 4, extensor indicis ; 

 5 and 6, long and short radial extensors of the carpus ; 7, the four divisions of the tendon 

 of the common extensor of tlie fingers ; 8, extePiSor minimi digiti ; 9, extensor carpi 

 ulnaris ; 10, flexor carpi radialis ; 11, flexor longus pollicis ; I'j, the first on the uhiar 

 side of the tendons of the flexor sublimis digitorum ; 13, the same of the flexor pro- 

 fundus ; 14, the median nerve ; 15, the palmar aponeurosis stretched across the annular 

 ligament ; 16, palmaris brevis ; 17, muscles of the ball of the thumb ; IS, muscles of 

 the little finger. 



Varieties. — A short additional extensor of one or more of the fingers has been 

 observed as an occasional variety by various anatomists since Albinus, and who 

 named it extensor brevis in the case of the index-finger with which it is most 

 frequently connected. But it appears from later observations to vary consider- 

 ably both in its origin, wliich may be from the radius or from a caipal or meta- 

 carpal bone, and in its insertion wlrich may be into onl}- one or into two or 

 ■more of the fingers ; thus forming an extensor brevis digiton^m. 



Nerves. — The anconeus. supinator longus , and extensor caii^i radialis longior 

 receive branches dii'ectly from the musculo-spii-al nerve ; the short radial extensor, 

 the ulnar extensor of the carpus, and the common extensor of the fingers, with 

 that of the little finger, are supplied from the radial division, while the supinator 



