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

## TL;DR

A rare muscular variation in the neck of a cadaver could impact surgical and medical procedures in the posterior cervical triangle.

## Contribution

The paper documents a unique fusion of trapezius and sternocleidomastoid muscles with surgical implications.

## Key findings

- A rare muscular configuration was found in the left cervical region of a 57-year-old woman's cadaver.
- The external jugular vein and clavicular branch of the supraclavicular nerves were affected by the anatomical variations.
- The anomaly is likely due to incomplete separation of the cucullaris anlage during embryological development.

## Abstract

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.