334 THE MUSCLES OF THE UPPER LIMB. 



The extensor longus pollicis muscle (ext. secundi internodii poll.), much 

 larger than the extensor brevis, which it overlaps, arises immediately below the 

 extensor ossis metacarpi from the outer division of the posterior surface of the 

 ulna for its middle third or more, and from the interosseous membrane for about an 

 inch opposite the lower part of the ulnar attachment. Its fibres end in a tendon 

 which passes through a separate compartment of the annular ligament, occupying 

 the narrow oblique groove in the middle of the posterior surface of the carpal end 

 of the radius, and is inserted into the base of the terminal phalanx of the thumb. 



Relations. The fleshy part of this muscle is covered by the extensors of the fingers and 

 the extensor carpi ulnaris. Its tendon becomes superficial immediately above the wrist, and 

 below the annular ligament crosses over those of the radial extensors. At the place of 

 crossing a communication is formed between the two synovial sheaths investing these 

 tendons. 



Varieties. 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. The most common occur in the extensor ossis metacarpi, and consist in a 

 more extensive cleavage of the tendon, or even of the whole muscle, into separate parts. The 

 insertion of the distinct tendons takes place either doubly into the first metacarpal bone, or in 

 part into the trapezium, or into the abductor or opponens pollicis muscles. The extensor brevis, 

 which is a muscle peculiar to man, is sometimes absent, being as it were fused with the 

 extensor ossis metacarpi. Its tendon is often united, and inserted in common with that of the 

 long extensor. Doubling of the extensor longus pollicis is not unfrequent, and the ulnar 

 portion of the muscle may then pass beneath the annular ligament with the extensor 

 communis digitorum. A slip from the tendon of the long extensor to the indicator is 

 occasionally seen. A rarer variety, representing a muscle normally existing in the dog and 

 many carnivora, is the presence of an additional extensor between the indicator and the 

 extensor longus pollicis, with a double tendon and insertion into both digits. 



The extensor indicis or indicator muscle arises from the outer division of the 

 posterior surface of the ulna for a variable extent below the extensor longus 



3 6 .5 Fig. 241. TRANSVERSE SECTION OP THE RIGHT HAND BETWEEN 



THE CARPUS AND METACARPUS. (Allen Thomson.) i 



a, 6, c, d, e, articular surfaces of the trapezium, trapezoid, 

 magnum and unciform bones ; a', palmar ridge of the trapezium; 

 e', unciform process ; between a' and ', the cut edge of the 

 annular ligament, sending a process towards the trapezoid at 11, 

 by which the tendon of the flexor carpi radialis is enclosed in 

 the groove of the trapezium ; 1 , tendon of the extensor ossis 

 mebacarpi pollicis ; 2, extensor brevis pollicis ; 3, extensor 

 longus ; 4, extensor carpi radialis longior ; 5, brevior ; 6, 

 index finger tendon of extensor communis digitorum ; 6', re- 

 maining tendons ; 7, extensor i indicis ; 8, extensor minimi 



digiti ; 9, extensor carpi ulnaris ; 10, flexor carpi radialis ; 11, flexor longus pollicis ; 12, the first on 



the ulnar side of the tendons of the flexor sublimis digitorum ; 13, the same of the flexor profundus ; 



14, median nerve ; 15, the palmar aponeurosis stretched across the annular ligament ; 16, palmaris 



brevis ; 17, short muscles of the thumb ; 18, muscles of the little finger. 



pollicis, and slightly from the interosseous membrane at its lower part. 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. 



Varieties. This muscle is but rarely absent. Its tendon is frequently double, and one of 

 the parts may pass, although rarely, to the thumb or ring finger, or more commonly (14 per 

 cent.) to the middle finger, leading to the formation of an extensor mcdii digiti, which may 

 occur also as a distinct muscle (2'5 per cent.) arising from the ulna or posterior ligament of 

 the wrist- joint below the indicator. Less frequently an extensor brevis digitorum tnanus is 

 present, arising from the back of the wrist-joint, or from the carpus, or from the bases of 

 some of the metacarpal bones, and sending tendons to one, two, or three fingers. Intermediate 



