<P> The brachial plexus is a network of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit . It supplies afferent and efferent nerve fibers to the chest, shoulder, arm and hand . </P> <P> The brachial plexus is divided into five roots, three trunks, six divisions (three anterior and three posterior), three cords, and five branches . There are five "terminal" branches and numerous other "pre-terminal" or "collateral" branches, such as the subscapular nerve, the thoracodorsal nerve, and the long thoracic nerve, that leave the plexus at various points along its length . A common structure used to identify part of the brachial plexus in cadaver dissections is the M or W shape made by the musculocutaneous nerve, lateral cord, median nerve, medial cord, and ulnar nerve . </P> <P> The five roots are the five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck . The brachial plexus emerges at five different levels; C5, C6, C7, C8, and T1 . C5 and C6 merge to establish the upper trunk, C7 continuously forms the middle trunk, and C8 and T1 merge to establish the lower trunk . Prefixed or postfixed formations in some cases involve C4 or T2, respectively . The dorsal scapular nerve comes from the superior trunk and innervates the rhomboid muscles which retract the scapula . The subclavian nerve originates in both C5 and C6 and innervates the subclavius, a muscle that involves lifting the first ribs during respiration . The long thoracic nerve arises from C5, C6, and C7 . This nerve innervates the serratus anterior, which draws the scapula laterally and is the prime mover in all forward - reaching and pushing actions . </P> <P> These roots merge to form thetrunks: </P>

Which nerve of the brachial plexus innervates the serratus anterior muscle
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