The muscle that is highlighted is the sternocleidomastoid, which arises from the sternum's manubrium and medial clavicle.
The sternoclavicular joint is formed by the clavicle's medial articulation with the sternum's manubrium (SC joint). An intra-articular disc is located between the clavicle and the sternum in this joint, which is encased in a fibrous capsule.
To keep the scapula in place and allow the arm to hang freely, the medial clavicle acts as a strut. The location of it is directly above the first rib. Its rounded medial end, also referred to as the sternal end, articulates with the manubrium of the sternum at the sternoclavicular joint (breastbone).
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