BONY THORAX
The skeletal framework of the thorax—the bony
thorax—consists of 12 pairs of ribs and their cartilages, 12 thoracic vertebrae
and intervertebral discs, and the sternum. The illustration also includes one
clavicle and scapula because these bones serve as important attachments for
some of the muscles involved in respiration. The sternum is made up of three
parts—the manubrium, body, and xiphoid process. The manubrium and body are not
in quite the same plane and thus form the sternal angle at their
junction, a significant landmark at which the costal cartilage of the second rib
articulates with the sternum. The superior border of the manubrium is slightly
concave, forming what is called the suprasternal
notch.
The costal cartilages of the first through seventh ribs ordinarily
articulate with the sternum and are called true ribs. The costal
cartilages of the eighth through tenth ribs ( false ribs) are usually
attached to the cartilage of the rib above, and the ventral ends of the cartilages
of the eleventh and twelfth ribs ( floating ribs) have no direct skeletal
attachment.
All of the ribs articulate dorsally with the vertebral column in such a
way that their ventral end (together with the sternum) can be raised slightly,
as occurs in inspiration. The articulations of the costal cartilages with the
sternum, except those of the first rib, are true or synovial joints that allow
more freedom of movement than there would be without this type of articulation.
The deep surface of the scapula (the subscapular fossa) fits against the
posterolateral aspect of the thorax over the second to seventh ribs, where, to
a great extent, it is held by the muscles that are attached to it. The acromion
process of the scapula articulates with the lateral end of the clavicle; this
acts as a strut to hold the lateral angle of the scapula away from the thorax.
On the dorsal surface of the scapula, a spine protrudes and continues laterally
into the acromion process. At its vertebral end, the spine flattens into a
smooth tri- angular surface with the base of the triangle at the vertebral
border. The spine separates the supraspinous fossa from the infraspinous fossa.
Three borders of the scapula are described—superior, lateral, and medial or
vertebral. On the superior border is a notch or incisura, and lateral to this,
the coracoid process protrudes anteriorly.
The lateral angle of the scapula presents a slight concavity, the glenoid
fossa, for articulation with the head of the humerus. At the superior end of
the glenoid fossa is the supraglenoid tuberosity, and at its inferior margin is
the infraglenoid tuberosity.
The clavicle articulates at its medial end with the superolateral aspect
of the manubrium of the sternum and at its lateral end with the medial edge of the
acromion process of the scapula. Its medial two-thirds are curved slightly anteriorly,
and its lateral third is curved posteriorly. Muscular attachments to the medial
and lateral parts of the clavicle leave its middle portion less protected and
thus readily subject to fracture.
The vertebral levels of the bony landmarks on the ventral aspect of the
thorax are variable and differ somewhat with the phase of respiration. In
general, the upper border of the manubrium is at the level of the second to
third thoracic vertebrae, the sternal angle opposite the fourth to fifth
thoracic vertebrae, and the xiphisternal junction at the level of the ninth
thoracic vertebra.
