Muscles There are generally twelve muscles in the<unk> , which can be further divided into superficial, intermediate, and deep. 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. The deep muscles arise from the distal part of the ulna and the surrounding interosseous membrane. The brachioradialis, flexor of the elbow, is unusual in that it is located in the<unk><unk> , but it is actually a muscle of flexor / anterior<unk>  of the<unk> . The anconeus, assisting in extension of the elbow joint, is by some considered part of the<unk><unk>  of the arm. The majority of muscles found in the<unk><unk>  are extrinsic , meaning that their origin has some distance from the part moved. The brachioradialis and the anconeus are considered intrinsic muscles because they both arise within the<unk>  and they both move the<unk> . Level Muscle Extrinsic/Intrinsic Innervation superficial brachioradialis intrinsic radial nerve superficial extensor carpi radialis longus extrinsic radial nerve superficial extensor carpi radialis brevis extrinsic radial nerve (deep branch) superficial extensor carpi ulnaris extrinsic radial nerve (as<unk>  interosseous nerve) superficial anconeus intrinsic radial nerve intermediate extensor digitorum extrinsic radial nerve (as<unk>  interosseous nerve) intermediate extensor digiti minimi extrinsic radial nerve (as<unk>  interosseous nerve) deep abductor pollicis longus extrinsic radial nerve (as<unk>  interosseous nerve) deep extensor pollicis longus extrinsic radial nerve (as<unk>  interosseous nerve) deep extensor pollicis brevis extrinsic radial nerve (as<unk>  interosseous nerve) deep extensor indicis extrinsic radial nerve (as<unk>  interosseous nerve) deep supinator intrinsic radial nerve (deep branch) Extensor tendon<unk> s Extensor tendons pass through the extensor retinaculum at wrist joint in 6 synovial sheaths, also referred to<unk> s. The supinator and the anconeus are the two extensor muscles in the<unk>  that do not pass through wrist extensor<unk> s. 558x558px # The first<unk>  locating the most radial is occupied by the extensor pollicis brevis and the abductor pollicis longus to insert to the thumb. # The second<unk>  is occupied by the two radial wrist extensors, the extensor carpi radialis longus and the extensor carpi radialis brevis. # The third<unk>  exclusively accommodates the extensor pollicis longus, which hooks around Lister's tubercle of radius and inserts to the thumb. # The fourth<unk>  is the largest of all. It is occupied by the extensors of the digits, the extensor digitorum communis and the extensor indicis proprius. The extensor indicis proprius usually runs and inserts onto the ulnar side of the extensor digitorum communis of the index finger. # The fifth<unk>  is occupied by the extensor digiti minimi, the extensor of the little finger. # The extensor carpi ulnaris passes through the sixth<unk>  to insert to the base of the fifth metacarpal bone. Innervation The muscles of the<unk>  are innervated by the radial nerve and its branches. The radial nerve arises from the<unk>  cord of the plexus. The somatomotor fibers of the radial nerve branch from the main radial nerve at the level of the radial groove of the humerus. Development In the early stage of development, the extensor precursor divides into 3 layers namely, superficial layer, radial layer and deep layer. The superficial group develops to become the extensor digitorum communis, the extensor carpi ulnaris and the extensor digiti minimi. The radial layer forms the extensor carpi radialis longus, the extensor carpi radialis brevis and the brachioradialis. The deep layer differentiates to become the abductor pollicis longus, the extensor pollicis longus and the extensor pollicis brevis. Phylogenetic origin Embryologic origin Representatives in humans Brachioantebrachial group Radial layer Brachioradialis Extensor carpi radialis longus Extensor carpi radialis brevis Supinator Superficial layer Extensor digitorum communis Extensor carpi ulnaris Extensor digiti minimi Antebrachiomanual group Deep layer (radial) Extensor pollicis brevis Abductor pollicis longus Deep layer (ulnar) Extensor pollicis longus Extensor indicis proprius Extensor digitorum brevis manus Manual group Variations The deep layer of the precursor extensor mass is known to be phylogenetically unstable and is undergoing evolution as high variability is seen in non-human primates. In humans, anomalous or additional muscles can be seen in small portion of population. Anomalous muscles in human extensor<unk>  are listed as follow: * Extensor medii proprius * Extensor indicis et medii communis * Extensor pollicis et indicis communis * Extensor carpi radialis tertius * Extensor digitorum brevis manus
Posterior compartment of the forearm
Ulnar claw
Seal finger
Finger cot
Boot 
Distal radioulnar articulation
Pelvic digit
Abductor wedge