|Derivation||Attached to the sternum, clavicle (cleido), and mastoid process.|
|Origin||The sternum and the medial third of the clavicle.|
|Insertion||The mastoid process on the skull.|
|Action||Bends the head laterally to the same side (drawing the ear toward the shoulder), and rotates the head to the opposite side (turning the face away). Both sides together roll the head towards the back.|
Sternocleidomastoid is an important form for establishing the shape of the neck. Its axis is an arc - often drawn downward - from the lateral base of the skull to the pit of the throat. Thus it connects back to front, lateral to medial, and high to low on the neck.
This main axis includes the origin and the insertion on the sternum. There is a second, flatter, somewhat subordinate form that drops down to the clavicle. Between the two heads there is a triangular depression that is visible on the surface.
Necks in beginners' drawings are prone to looking like the base of the Eiffel Tower - curved inward on both sides. This comes from joining the outer curves of sternocleidomastoid and trapezius. By separating these curves and drawing sternocleidomastoid from its insertion to its origins, a convincing shape for the neck and upper shoulders can be established with a few lines. (Artists should become suspicious of themselves when inclined to draw concave shapes on the figure. This is discussed further in the last section.)