Ribs

Ribcage
The ribcage is composed of twelve pairs of ribs which articulate with the vertebrae of the spinal column and the sternum (with some exceptions) to create the thoracic cavity.  The ribs themselves are flat, curved bones, all of which articulate posteriorly with the vertebrae at costal facets.  The upper seven pairs of ribs also articulate anteriorly with the sternum, while the lower five pairs do not.  The eighth, ninth, and tenth pairs are attached by costal cartilage to the seventh pair, while the eleventh and twelfth pairs are not attached anteriorly at all.  The ribcage provides a sturdy support for the thorax, protecting the heart, lungs, and other important internal organs.  The arrangement of the ribs, however, allows the expansion of the ribcage, as during breathing.  A large number of muscles and ligaments attach to the ribs which, with the flexibility of the ribcage, allow the thorax to be both supple and strong.

The true ribs are the upper seven pairs in the ribcage.  They are called true ribs because they articulate anteriorly to the sternum, distinguishing them from false ribs, which do not.  The true ribs are also connected anteriorly to the spinal vertebrae.  In both cases, the connection is cemented by costal cartilage.

The lower five pairs of ribs are called false ribs, because they do not directly articulate with the sternum.  Instead, the eighth, ninth, and tenth ribs are joined to the seventh rib by cartilaginous tissue.  The eleventh and twelfth pairs of ribs (the last two pairs of false ribs) are also called floating ribs, because they do not connect anteriorly to any other rib or the sternum.

Costal (rib) cartilage is the connective tissue which attaches the ribs anteriorly (in the front) to the sternum and posteriorly (in the back) to the vertebrae of the spine.  The costal cartilage is similar to glue, but is more resilient, with a smooth, shiny texture.

The vertebrae are irregularly shaped bones which stack together to form the spinal column.  The vertebrae are connected together by ligaments and muscles which control the degree of flexibility of the spine.  The vertebrae are cushioned from each other by cartilage disks which act as shock absorbers to protect the vertebrae in the spine.  The vertebrae may be separate (cervical, thoracic, and lumbar vertebrae), semi-articulated (as in some coccygeal vertebrae), or fused (as in the sacrum and coccyx).  The typical vertebra has a body of solid bony material, which supports the weight of the spine, and an arch, which forms the vertebral foramen.  It is the adjoining vertebral foramina which creates a canal down through the spinal column which houses and protects the spinal cord.  The thoracic vertebrae feature facets to which the ribs attach, called costal facets (because of their relation to the ribs).

The sternum is a flat, blade-like bone located at the center of the chest.  It serves as the anterior (forward) site of articulation for the ribs via cartilaginous connections, called costal cartilage.  The pectoralis major also anchors to the sternum, giving the shoulder joint much of its strength during flexion of the arm.  The sternum features two articulations in addition to its costal (rib) articulations.  One of these, called the manubriosternal joint, is between the body (middle plate) of the sternum and the broader upper section, called the manubrium.  The manubrium of the sternum articulates with the clavicles and the sternocleidomastoid, sternohyoid, and sternothyroid muscles connect here.  The lower articulation is called the xiphisternal joint, and is between the body of the sternum and a small, teardrop-shaped bone called the xiphoid (pronounced "zy'-foid") process.  The xiphoid process anchors the rectus abdominis, the transverse thoracic, and the diaphragm muscles, responsible for much of the muscular expansion and contraction of the abdomen.

The xiphoid (pronounced "zy'-foid") process is a teardrop-shaped bone which articulates with the body of the sternum at the xiphisternal joint.  This process anchors the rectus abdominis, the transverse thoracic, and the diaphragm muscles, responsible for much of the muscular expansion and contraction of the abdomen.

The manubrium of the sternum is the broad, disc-like, upper part of the sternum.  It articulates with the body of the sternum at the manubriosternal joint.  The manubrium of the sternum articulates with the clavicles and the sternocleidomastoid, sternohyoid, and sternothyroid muscles connect here.  The top of the manubrium features the small jugular notch which admits passage of
the jugular vein along the bone.

The jugular notch is located at the top of the manubrium of the sternum.  This notch admits passage of the jugular vein along the bone.

The clavicle, or collarbone, is a long, slightly curving bone which forms the frontal (anterior) part of each shoulder (pectoral) girdle.  Located just above the first rib on each side of the ribcage, clavicles attach to the sternum in the middle of the chest, and laterally to the acromion of the scapula (forming the acromioclavicular joint).

The scapula (shoulder blade) is a rougly triangular bone which, with the clavicle, forms the pectoral, or shoulder, girdle.  The humerus, or upper arm bone, articulates with the scapula to form the shoulder joint.  This articulation takes place at the glenoid cavity, located at the upper, lateral angle of the scapula.  The posterior of the scapula features a laterally running spine ,which separates the posterior surface into two unequal areas.  This spine continues laterally and projects in the coracoid process and the acromion (which articulates with the medial end of the clavicle).  Both of these projections serve as sites of attachment for connective tissue, and the spine and acromion anchor the trapezius and deltoids, specifically.  These connections give the pectoral girdle a high degree of both flexibility and strength.

IAP: anterior
IAP: posterior

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