Rare Bilateral Axillary Arch With Its Novel Insertion Into the Deltoid and Unilateral Supernumerary Head of the Sternocleidomastoid Muscle in a Single Cadaver.

Two rare and clinically significant anatomical variations were observed in a single cadaver during routine dissection at the Department of Cellular Biology and Anatomy of Louisiana State University (LSU) Health Shreveport in Shreveport, Louisiana. We report a rare case of a bilateral axillary arch muscle. On the right side, the muscle originated from the latissimus dorsi and merged with the short head of the biceps brachii, deltoid, and coracobrachialis muscles. On the left, it originated from the latissimus dorsi but was inserted into the fascia between the coracobrachialis and deltoid muscles. To our knowledge, this branching pattern finding has not been previously described in the literature. This anatomical variation may influence upper limb mobility and pose potential challenges during axillary surgical procedures and the diagnosis of compression syndromes. Additionally, in the same cadaver, an accessory head of the sternocleidomastoid muscle on the left side, originating from the middle third of the clavicle, was observed, which could impact neck mobility. Recognition of both variations holds clinical significance, contributing to advancements in anatomical knowledge relevant to surgery, radiology, and physical therapy.