(b) Anteriorly, the upper 10 pairs of ribs are attached directly or indirectly
to the sternum. The indirect attachments are made through costal cartilages to the ribs
(c) It is important to note that both the posterior and anterior articulations
are located essentially in the midline of the body, back and front.
(3) Costal cartilages. The costal cartilages are bars of cartilage of varying
lengths. Since costal cartilages are elastic, they can be twisted (deformed) and
returned to their original shape.
b. Sternum. The sternum is located in the midline anteriorly, immediately
beneath the skin. (Since the sternum is a flat bone with hematopoietic (blood-forming)
red marrow and is so close to the surface of the body, it is a convenient location for
taking a sample of hematopoietic tissue for clinical examination--the sternal punch.)
(1) The sternum is made up of three parts--the manubrium above, the body
as the main portion, and the xiphoid process below.
(2) Where the manubrium articulates with the top of the body of the sternum
is a sternal angle (Louis' angle). The sternal angle is important in costal breathing,
since it allows for greater expansion of the rib cage. (In the clinic, the sternal angle is
important as a landmark. It marks the site of the second rib and is used to identify
locations on the chest wall.)
c. Thoracic Vertebrae. Posteriorly, there are 12 thoracic vertebrae, joined by
intervertebral discs. Their curvature, the thoracic curvature, is concave anteriorly.
During breathing, this curvature straightens and thus increases the expansion of the rib
d. Segmentation. The segmentation of the thorax is produced by both the
intervertebral discs and the intercostal spaces between adjacent ribs. Such
segmentation of the rib cage allows motion to take place, especially bending to the right
e. Intercostal Muscles. The intercostal spaces are filled by two layers of
intercostal muscles. The intercostal muscles extend from the vertebrae behind to the
sternum in front. A strengthening "plywood effect" is created by the arrangement of the
two layers at a right angle to each other. Therefore, these muscles help to maintain the
"solid-wall" condition of the thorax. For this reason, a pressure gradient can be
maintained between the inside and outside of the thorax.
f. Skeletal Muscles Attached to the Rib Cage. Various skeletal muscles are
attached to the rib cage. Some extend from above and draw the rib cage upward.
Others extend from below and draw the cage downward.