The sterno-cleido-mastoid (French: muscle sterno-cléido-mastoïdien), or sterno-mastoid muscle, named from its attachments, is a strong ribbon-shaped and, bifurcated below, and somewhat constricted in its middle third.
Clavicular head : the upper part of the anterior surface of the inner third of the clavicle.
- Along the anterior border and the upper part of the outer surface of the mastoid process of the temporal bone;
- the outer half of the superior nuchal line of the occipital bone.
The sternal head is a rounded but flat tendon ; the clavicular is sortly fleshy and partly tendinous. After a course of about an inch, the sternal head expands into a flat muscle, which conceals the greater part of the clavicular portion, and, passing upwards, outwards, and backwards, is spread over the whole bone of the upper attachment. Frequently it is so separate from the clavicular head, that they might very fairly be considered to form two muscles. The clavicular head soon becomes entirely fleshy and ascends more directly. At first it is separated by a small interval from the sternal head, corresponding to a part of the sterno-clavicular joint, from which it sometimes receives a few fibers of origin ; when they have joined, it passes beneath the sternal head to its insertion, which is chiefly the lower part of the outer surface of the mastoid process. The whole insertion in front is composed of short tendinous fibers, and behind of a thin aponeurosis. As the whole muscle has a very wide range of action, nearly the whole length of its fibers is fleshy. The sternal head is a little longer than the other, but has little if any more range of movement. Hence it is tendinous.
- The spinal accessory nerve, which, while traversing the deep surface of the muscle at the junction of its upper and middle thirds, sends filaments to it;
- the cervical plexus through the anterior primary branches ( the second and third cervical nerves, which enter the upper part of its deep surface.
Anterior and Lateral Cervical Muscles.
- To flex laterally the head and neck, so as to draw the side of the head towards the shoulder.
- To rotate the face towards the opposite side. Of the two parts of the muscle, the cleido-mastoid portion is more concerned in lateral flexion, the sterno-mastoid in rotation. The combination of all these movemen may be very well seen in a case of wry-neck, which results from the permanent contraction of this muscle.
- When both muscles act, to flex the head and neck upon the thorax, at the same time raising slightly the chin, which is therefo carried horizontally forwards.
- To raise the sternum and inner end of the‚cllavicle. This action may sometimes be seen in patients with paralysis of all the parts beneath the cervical region, when the only nerves available for respiratory movements are the phrenic and those which supply certain of the muscles of the neck.
- If the head be much thrown back, the two sterno-mastoids may be used to increase the extension.
Superficially, the deep cervical fascia and integuments, the platysma hyoides, external jugular vein, and many of the superficial branches of the cervical plexus, and the glandule concatenatae ; deeply, the rectus capitis anticus major, omo-hyoid, sterno-hyoid and sterno-thyroid, the posterior belly of the digastric, the splenius capitis, levator anguli scapulae, and three scalene muscles ; the common, external, and internal carotid and subclavian arteries, with several branches of the external carotid, the internal jugular, facial, thyroid, anterior jugular, and other veins ; the spinal accessory and hypoglossal nerves, the cervical and upper part of the brachial plexuses with many of their branches, the lateral lobe of the thyroid gland, and numerous deep cervical lymphatic glands.
The clavicular origin may extend farther outwards upon the collar bone, besides its insertion into the mastoid process, this head may have an attachment to the superior nuchal line, called the cleido-occipital. An extension of the sternal head has been observed arising from the costal cartilages as low as that of the fifth rib. Slips sometimes pass from the upper part of the muscle to the angle of the jaw, the pharynx, be auricle, or the upper attachment of the trapezius.