When Muscle Lines Blur: An Unusual Fusion of Trapezius and Sternocleidomastoid With Surgical Relevance.

During the routine dissection of the cadaver of a 57-year-old Indian woman, a rare constellation of muscular variations was observed in the left cervical region. A well-developed sternocleido-occipital muscle with distinct sternal and clavicular heads arises from the external occipital protuberance, the medial third of the superior nuchal line, the ligamentum nuchae, and the spinous processes of C7-T12. Adjacent to it, a separate cleido-occipital slip merged posteriorly with the main muscle, while an accessory trapezial bundle coursed inferolaterally to the clavicle. Notably, the external jugular vein (EJV) passes between the trapezial bundle and the principal muscle, and a musculotendinous arch overlies the clavicular branch of the supraclavicular nerves, potentially causing compression. These variants collectively altered the configuration of the posterior cervical triangle without evidence of trauma or surgical change. Embryologically, the anomaly likely reflects incomplete separation of the cucullaris anlage, and phylogenetically, persistence of a primitive muscular pattern. Awareness of such configurations is vital, as they may affect surgical, radiological, and anesthetic interventions involving the posterior triangle.