sternocleidomastoïdeus

The sternocleidomastoideus, commonly known as the sternocleidomastoid muscle, is a prominent paired muscle of the neck. It has two heads: a sternal head arising from the manubrium of the sternum and a clavicular head arising from the medial third of the clavicle. Both heads insert at the mastoid process of the temporal bone and the adjacent superior nuchal line. The muscle lies superficial to many deeper neck structures and helps define the contour of the neck. It forms part of the posterior border of the anterior and posterior triangles of the neck.

Anatomical relations include its position as a key landmark in the neck, separating the anterior from the posterior triangles. It is overlapped by the platysma anteriorly and covered by cervical fascia, with deeper muscles and vessels lying beneath. The sternocleidomastoideus is typically visible when the head is turned and the neck is relaxed.

Functionally, contraction of the sternocleidomastoideus is bilateral or unilateral. Unilateral contraction rotates the head to face the opposite side and tilts the head toward the same shoulder. Bilateral contraction flexes the neck, with the upper fibers extending the head at the atlanto-occipital joints. When the head is fixed, it can assist in elevating the sternum and clavicle during deep inspiration.

Innervation is provided primarily by the spinal accessory nerve (cranial nerve XI), with proprioceptive input from C2–C3. Blood supply comes from branches of the superior thyroid, occipital, and posterior auricular arteries, with venous drainage into the external jugular system. Clinically, the muscle can be involved in conditions such as torticollis or SCM strain and is a common landmark in neck examinations and procedures.