212 MYOLOGY, OR ANATOMY OF MUSCULAR SYSTEM 



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fascia connected with the triceps; rises by a narrow tendon 

 from a fossa on the inner and posterior part of the external 

 condyle; upper fibers are transverse, the rest pass obliquely 

 down and in to the radial aspect of the olecranon and adjacent 

 upper third of the ulna. As a rule, its superior fibers are con- 

 tinuous with those of the internal head of the triceps. A 

 bursa is found under its tendon of origin, not in children. 



DEEP LAYER. 1. M. Supinator Brevis. Origin, external 

 lateral ligament, orbicular ligament, supinator ridge, oblique 

 line of the ulna, and for a short distance on the outer border 

 of the ulna, from the fascia covering it, which is connected 

 with the external condyle; it regularly consists of two layers 

 separated by the posterior interosseous nerve; the superficial 

 one rises by aponeurotic fibers, the other by muscular. The 

 fibers pass sling-like around the upper part of the radius to 

 be inserted into a third of its length, limited by the anterior 

 and posterior oblique lines, to its neck and elbow-joint capsule. 



Anterior fibers may have insertion into the biceps tendon 

 or tuberosity of the radius. 



2. M. Extensor Ossis Metacarpi Pollicis (abducens pollicis 

 longus). Origin, upper part of the outer division of the pos- 

 terior surface of the ulna below the supinator brevis, from 

 the middle third of the posterior surface of the radius and inter- 

 osseous membrane between. Its tendon passes over those of 

 the radial extensors, and is inserted into the radial side of the 

 base of the metacarpal bone of the thumb, and commonly by a 

 slip into the trapezium, its tendon usually splitting. 



3. M. extensor longus pollicis (extensor securidi internodii 

 pollicis) rises below the extensor ossis on the middle third of 

 the ulna and from the interosseous membrane for about 1 

 inch; its tendon passes over the radial extensors, and is inserted 

 into the dorsal aspect of the base of the distal phalanx of the 

 thumb. There is a tendency for it to divide into three parts, 

 as in case of the extensor communis tendons, but all three 

 converge to the base of the distal phalanx. 



4. M. Extensor Indicis Proprhis (M. indicator). Origin, 

 from the ulna below the extensor longus pollicis, and slightly 

 from the interosseous membrane and fascia over the extensor 

 carpi ulnaris; unites with the common extensor tendon for 

 the index, and forms the usual insertion. This and the extensor 



