The radial nerve arises from the posterior cord of the plexus. The muscles of the posterior compartment of the forearm are innervated by the radial nerve and its branches. The extensor carpi ulnaris passes through the sixth compartment to insert to the base of the fifth metacarpal bone.The fifth compartment is occupied by the extensor digiti minimi, the extensor of the little finger.The extensor indicis proprius usually runs and inserts onto the ulnar side of the extensor digitorum communis of the index finger. It is occupied by the extensors of the digits, the extensor digitorum communis and the extensor indicis proprius. The fourth compartment is the largest of all.The third compartment exclusively accommodates the extensor pollicis longus, which hooks around Lister's tubercle of radius and inserts to the thumb.The second compartment is occupied by the two radial wrist extensors, the extensor carpi radialis longus and the extensor carpi radialis brevis.The first compartment locating the most radial is occupied by the extensor pollicis brevis and the abductor pollicis longus to insert to the thumb.Radial nerve (as posterior interosseous nerve) The brachioradialis and the anconeus are considered intrinsic muscles because they both arise within the forearm and they both move the forearm. The majority of muscles found in the posterior compartment are extrinsic, meaning that their origin has some distance from the part moved. The anconeus, assisting in extension of the elbow joint, is by some considered part of the posterior compartment of the arm. The brachioradialis, flexor of the elbow, is unusual in that it is located in the posterior compartment, but it is actually a muscle of flexor / anterior compartment of the forearm. The deep muscles arise from the distal part of the ulna and the surrounding interosseous membrane. Most of the muscles in the superficial and the intermediate layers share a common origin which is the outer part of the elbow, the lateral epicondyle of humerus. There are generally twelve muscles in the posterior compartment of the forearm, which can be further divided into superficial, intermediate, and deep. 2.4 Misdiagnosis of rare anatomical variants.
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