The best way for us to learn about the upper extremity is to begin at the very beginning, right up here. We'll start by looking at the bones of the shoulder region: the clavicle, the scapula and the humerus. Then we'll look at the joints that let them move, and the muscles, which make them move. Lastly we'll look at the principal blood vessels and nerves in the region. First, the bones.
BONES, JOINTS AND LIGAMENTS
The bones that connect the upper extremity to the trunk are the clavicle, or collar bone, and the scapula, or shoulder blade. The parts of them that we can feel beneath beneath the skin can be seen in this dissection: here's the spine of the scapula, here's the clavicle. In the dry skeleton, here's the clavicle, here's the scapula.
The proximal long bone of the upper extremity, the humerus articulates with the scapula at the shoulder joint. The scapula and clavicle articulate with the bones of the thorax at one point only, here, at the sternoclavicular joint.
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 linkage, the scapula is held onto 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.
Looking at the clavicle from above we can see that it's slightly S-shaped, with a forward curve to its medial half. At its medial end this large joint surface articulates with the sternum. At the lateral end this smaller surface articulates with the scapula. On the underside, massive ligaments are attached, here laterally and here medially.
The scapula is a much more complicated bone. The flat part, or blade, is roughly triangular with an upper border, a lateral border, and a medial border. The blade isn't really flat, it's a little curved to fit the curve of the chest wall.
This smooth concave surface is the glenoid fossa. It's the articular surface for the shoulder joint. Above and below the glenoid fossa are the supraglenoid tubercle, and the infraglenoid tubercle, where two tendons are attached as we'll see.
A prominent bony ridge, the spine of the scapula, arises from the dorsal surface, and divides it into the supraspinous fossa, and the infraspinous fossa. At its lateral end the spine gives rise to this flat, angulated projection, the acromion, which stands completely clear of the bone. The clavicle articulates with the scapula here, at the tip of the acromion. This other projection, looking like a bent finger, is the coracoid process.
Here's how the clavicle and the scapula look in the living body. Round the edge of the shallow glenoid fossa, a rim of fibrocartiilage, the glenoid labrum, makes the socket of the shoulder joint both wider and deeper. This flat ligament, the coracoacromial ligament, joins the coracoid process to the acromion. Here's the acromioclavicular joint. Two strong ligaments, the trapezoid in front and the conoid behind, fix the underside of the clavicle to the coracoid process. There's very little movement at the acromio-clavicular joint.
As we've seen, the medial end of the clavicle articulates with the sternum at the sterno-clavicular joint.
Strong ligaments between the clavicle and the sternum and between the clavicle and the underlying first rib, keep the two bones together but permit an impresssive range of motion: up and down, and backward and forward.