An anomalous cleido-occipitalis muscle.

In a Japanese female cadaver, the occipital portion of the trapezius muscle (the cleido-occipitalis) was separated from the rest of the muscle by a triangular gap that resulted from a deficiency of the upper two thirds of the cervical portion on both sides. The right cleido-occipitals blended distally with the cervical fibers of the trapezius and had a normal insertion into the clavicle. The left cleido-occipitalis intermingled partially with the reduced cervical portion, and some of the conjoint fibers formed an independent accessory slip whose tendon inserted into the posterior aspect of the junction of the middle and medial thirds of the clavicle. The other conjoint fibers inserted into the clavicle and scapula as usual. This anomalous morphology may be attributed to a secondary degeneration of a portion of the trapezius anlage concomitant with an abnormal segregation of another portion during development.