Keiko Ogami-Takamura; Kazunobu Saiki; Daisuke Endo; Kiyohito Murai & Toshiyuki Tsurumoto

Variations in the triceps brachii muscle are uncommon, and especially limited reports exist on the accessory heads of tendinous origin that attach near the upper medial part of the humerus. During anatomical training at Nagasaki University School of Medicine, the accessory head of the triceps brachii muscle was observed on the right upper arm of a 72-year-old Japanese female. It arose tendinously from the medial side of the upper humerus, then formed a muscle belly and joined the distal side of the long head. This accessory head had independent nerve innervation, and the innervating nerve branched from a bundle of the radial nerve, which divided the nerve innervating the long head and the posterior brachial cutaneous nerve. The origin of the innervation of the accessory head was the basis for determining that this muscle head was an accessory muscle to the long head of the triceps brachii muscle. Embryologically, we discuss that part of the origin of the long head of the triceps brachii muscle was separated early in development by the axillary nerve and the posterior brachial circumflex artery, and it slipped into the surgical neck of the humerus and became fixed there. The accessory head crossed the radial nerve and deep brachial artery. When clinicians encounter compression of the radial nerve or profunda brachii artery, they should consider the presence of accessory muscles as a possible cause.

KEY WORDS: Anatomic variation; Cadaver; Radial nerve; Triceps brachii muscle.

OGAMI-TAKAMURA, K.; SAIKI, K.; ENDO, D.; MURAI, K. & TSURUMOTO, T. The Accessory muscular head of the triceps brachii muscle with tendinous attachment. Int. J. Morphol., 42(1):17-20, 2024.