Extensor Carpi Radialis Brevis and Extensor Carpi Radialis Longus

In lower mammals the two extensores carpi radiales are represented by one muscle; human cases have been reported in which the two muscles were fused, the common belly having two or three tendons inserted into the metacarpal bones.

A trigastric radialis longus has been reported inserting partially onto the base of the first metacarpal and partially onto the base of the second metacarpal. Incomplete fusion frequently occurs, effected by cross slips from one muscle to the other. The longus and brevis are noted for exchange of fleshy and tendinous slips. These connecting slips may be replaced by distinct muscles connected with one or both radial extensors.

Extensor carpi radialis intermedius is a rare muscle that arises independently from the epicondyle of the humerus between the two usual muscles and is inserted into the second and third metacarpals. More frequently, the accessory muscle is represented by a slip from one or both of the radial extensors and is inserted, in the same way, onto the second or third metacarpal or onto both of these bones. Extensor carpi radialis accessorius is an additional muscle sometimes seen arising from the humerus, with or below the long radial extensor, and inserting most frequently onto the metacarpal bone of the thumb. It is represented at times by a slip from the tendon of extensor longus. The muscle of the two extensors are usually separated, but, more commonly, it is one of the tendons that is split into two or sometimes three tendons prior to insertion. The tendons may be attached to the second and third metacarpal bones. Occasionally, a slip also passes to the fourth metacarpal or to the trapezium.

Extensor carpi radialis longus may be joined to abductor pollicis longus or to some of the dorsal interosseous muscles.

Syn.: Radialis extensor brevis s. secundus, Kurzer ausserer Speichenmuskel, Kurzer Speichenstrecker, Second radial externe (Cruveilhier).

Radialis extensor longus, Langer ausserer Speichenmuskel, Langer Speichenstrecker, Premier radial externe (Cruveilhier).

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