Osteology: Chest and Shoulder Girdle

The manubrium is the wide triangular superior portion of the sternum, and is can be either fused or unfused with the corpus sterni.  The superior margin of the manubrium bears a jugular notch along the midline, and the superior-lateral corners of the manubrium bear clavicular notches which receive the distal end of the clavicle.  Just inferior to these notches is the first costal notch for receiving the costal cartilage from the first rib.  The inferior-lateral corners of the manubrium bear a costal notch for the second costal cartilage.  The articulation of the second costal cartilage is usually shared between the manubrium and the corpus sterni, the long rectangular middle portion of the sternum.

The corpus sterni is dorsal-ventrally flattened.  The inferior portion is usually wider (medial-laterally) than the superior portion.  The lateral margins bear facets for the costal cartilages of ribs 2-7.

The most inferior portion of the sternum is the xiphoid process.  This process begins as cartilage and often never fully ossifies.  When ossification occurs, the xiphoid process often fuses to the corpus sterni.

The clavicle, along with the scapula, forms part of the shoulder girdle which affixes the upper limb to the axial skeleton. The characteristic S-shape of the clavicle results from the curvature of the shaft which curves posteriorly near its distal articulation with the acromion process of the scapula before changing course to curve ventrally as it approaches its proximal articulation with the clavicular facet of the manubrium. The clavicle functions as a bony strut which holds the shoulder joint away from the midline of the body, and is unique to primates.

The sternal facet is a broad irregularly-shaped articular surface on the proximal end of the clavicle.  The sternal facet has a distal extension that terminates in the costal facet, which forms a tuberosity in more heavily muscled individuals.  The subclavian groove on the inferior surface of the clavicle provides the attachment for the subclavius muscle.  The conoid tubercule is the attachment site for the conoid ligament, located on the ventral border of the distal end of the clavicle.  The oblique ridge runs laterally from the conoid tubercle, and serves as the attachment for the trapezoid ligament.  The conoid and trapezoid ligaments firmly attach the clavicle to the coronoid process of the scapula.

Siding the Clavicle

Even fragmentary clavicles can be readily sided.  The distal portion of the clavicle is extremely compressed superior-inferiorly whereas the proximal portion is circular or ovoid.  The superior surface of the clavicle is relatively flat and has few diagnostic features, while the inferior aspect is interrupted by several muscle attachments, especially in more robust individuals.  The conoid tubercle is located on the inferior-ventral margin of the distal end of the clavicle.

Along with the clavicle, the scapula forms the shoulder girdle, which anchors the arm to the axial skeleton.  The scapula is commonly known as the shoulder blade. As this common name suggests, the scapula takes the form of a triangular, ventral-dorsally compressed blade.  Much of the bone in the scapula is literally paper-thin and translucent.

Several of the borders and angles of the scapular triangle are given names according to their anatomical positions. The lateral and vertebral borders form the lateral and medial borders of the triangle.  The superior and inferior angles of the triangle are located in the superior-medial and inferior corners of the triangle.

The glenoid fossa is the laterally facing kidney-shaped articular surface located on the superior-lateral corner of the scapular triangle.  The glenoid fossa receives the head of the humerus to form the shoulder joint.  The coracoid process begins just superior-ventrally from the glenoid fossa and runs ventrally and laterally to provide the attachment point for a variety of muscles acting on the shoulder joint.  The scapular notch (sometimes a foramen) is a semicircular notch in the superior border of the scapula just medial to the coracoid process.  This notch transmits the suprascapular nerve.

Viewed from the dorsal aspect, the massive scapular spine begins approximately ¼ of the way down the vertebral border and courses superior-laterally to terminate in the ventral-laterally projecting acromion process.  The clavicle articulates with the acromion process on its ventral-medial corner, which bears a small articular facet.  The scapular spine divides the dorsal aspect of the scapula into a supraspinous fossa and an infraspinous fossa.  These fossae house muscles of the same name which act on the elbow joint.  The ventral aspect of the scapula is dominated by the massive but shallow subscapular fossa.  Several lateral lines or oblique ridges traverse the subscapular fossa obliquely from superior-lateral to inferior-medial.

Siding the Scapula

Intact scapula are relatively easy to side.  The glenoid fossa is lateral, the scapular spine is dorsal, and both the acromion and coracoid processes project superiorly.  Because portions of the scapula are so thin, scapulae are very easily fragmented.  The glenoid process and the roots of the acromion and coracoid processes are formed from relatively thick bone, and are often preserved in isolation from the rest of the scapula. The kidney shape of the glenoid is useful for siding because the kidney is more constricted superiorly, and the concave margin is ventral.  The acromion originates on the dorsal side of the scapula from the scapular spine, while the coracoid originates along the superior margin.  The lateral border of the scapula is much more robust and well-buttressed than the vertebral border.  The dorsal surface of the scapula is convex, while the ventral surface is concave.  The superior-lateral to inferior-medial course of the oblique lines on the ventral surface of the scapula are also useful for siding fragments.