Next we’ll take a further look at the first rib, a landmark structure where the thorax becomes continuous with the neck.
The first rib is the most tightly curved of all the ribs. It’s also the broadest of the ribs. When seen from the side, its upper border lies in a plane that’s about 30° from the horizontal. In addition, when seen from in front its flat upper surface slopes downward, also at about 30°.
The costal cartilage of the first rib articulates with the manubrium of the sternum not at the top, but lower down at its broadest part. The first costal cartilage is short and massive. It hardly permits any movement, so the two first ribs, together with the manubrium, move up and down together as one solid arch.
Here’s a dissection of the manubrium, and the two first ribs, with all the other ribs removed. Here’s the movement these structures make, when we take a deep breath in, and out.
The upper part of the thorax is almost completely surrounded by the muscles of the shoulder region, which arise from the ribs, and also from the vertebrae. These muscles are shown in Volume 1 of this atlas. Just to appreciate how greatly the structures of the shoulder affect the shape of the upper part of the body, we'll add to our picture at this point the bones of the shoulder region: the clavicles and the scapulae.
Here’s the clavicle, or collar bone, here’s the scapula, or shoulder blade. These two bones articulate with the bones of the thorax at one point only, here. The medial end of the clavicle articulates with the highest point on the manubrium, forming the sterno-clavicular joint.
It’s easy to palpate the clavicle. Here’s its medial end. The first rib is difficult to palpate. That's because it lies both below and behind the clavicle, and also because there’s a thick layer of muscle in front of it.
The lateral end of the clavicle articulates with this projection on the scapula, the acromion, forming the acromio-clavicular joint. Apart from this one very movable bony attachment, the scapula is held on to the body entirely by muscles.
It’s thus capable of a wide range of movement, upward and downward, and also forward and backward around the chest wall.