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In human and most vertebrate anatomy, the sternum or breastbone is a long, flat bone located in the center of the thorax (chest). It
connects to the rib bones via cartilage,
forming the rib cage with them, and thus helps to protect the lungs and heart from physical trauma.
The sternum is usually cut open (a sternotomy) to gain access to the thoracic contents when performing cardiothoracic surgery.
Overview
The sternum (Figs. 1 to 3) is an elongated, flattened bone, forming the middle portion of the anterior wall of the thorax. Its
upper end supports the clavicles, and its margins articulate with the cartilages of the first seven pairs of ribs. It consists of
three parts, named from above downward, the manubrium, the body or gladiolus, and the xiphoid process; in early life the body
consists of four segments or sternebrœ. In its natural position the inclination of the bone is oblique from above,
downward and forward. It is slightly convex in front and concave behind; broad above, becoming narrowed at the point where the
manubrium joins the body, after which it again widens a little to below the middle of the body, and then narrows to its lower
extremity. Its average length in the adult is about 17 cm., and is rather greater in the male than in the female.
Figure 1 : Anterior surface of sternum and costa cartilages.
Manubrium
('manubrium sterni') The manubrium is of a somewhat quadrangular form, broad and thick above, narrow below at
its junction with the body.
Surfaces
Its anterior surface, convex from side to side, concave from above downward, is smooth, and affords attachment on either side
to the sternal origins of the Pectoralis major and Sternocleidomastoideus. Sometimes the ridges limiting the attachments of these
muscles are very distinct. Its posterior surface, concave and smooth, affords attachment on either side to the Sternohyoideus and
Sternothyreoideus.
Figure 2 : Posterior surface of sternum. |
Figure 3 : Lateral border of sternum. |
Borders
The superior border is the thickest and presents at its center the jugular or presternal notch; on either side of the notch is
an oval articular surface, directed upward, backward, and lateralward, for articulation with the sternal end of the clavicle. The
inferior border, oval and rough, is covered in a fresh state with a thin layer of cartilage, for articulation with the body. The
lateral borders are each marked above by a depression for the first costal cartilage, and below by a small facet, which, with a
similar facet on the upper angle of the body, forms a notch for the reception of the costal cartilage of the second rib. Between
the depression for the first costal cartilage and the demi-facet for the second is a narrow, curved edge, which slopes from above
downward and medialward.
Body
('corpus sterni; gladiolus') The body, considerably longer, narrower, and thinner than the manubrium, attains
its greatest breadth close to the lower end.
Surfaces
Its anterior surface is nearly flat, directed upward and forward, and marked by three transverse ridges which cross the bone
opposite the third, fourth, and fifth articular depressions. 18 It affords attachment on either side to the sternal origin of the
Pectoralis major. At the junction of the third and fourth pieces of the body is occasionally seen an orifice, the sternal
foramen, of varying size and form. The posterior surface, slightly concave, is also marked by three transverse lines, less
distinct, however, than those in front; from its lower part, on either side, the Transversus thoracis takes origin.
Borders
The superior border is oval and articulates with the manubrium, the junction of the two forming the sternal angle (angulus
Ludovici 19). The inferior border is narrow, and articulates with the xiphoid process. Each lateral border [Fig. 3], at its
superior angle, has a small facet, which with a similar facet on the manubrium, forms a cavity for the cartilage of the second
rib; below this are four angular depressions which receive the cartilages of the third, fourth, fifth, and sixth ribs, while the
inferior angle has a small facet, which, with a corresponding one on the xiphoid process, forms a notch for the cartilage of the
seventh rib. These articular depressions are separated by a series of curved interarticular intervals, which diminish in length
from above downward, and correspond to the intercostal spaces. Most of the cartilages belonging to the true ribs, as will be seen
from the foregoing description, articulate with the sternum at the lines of junction of its primitive component segments. This is
well seen in many of the lower animals, where the parts of the bone remain ununited longer than in man.
Xiphoid Process
('processus xiphoideus; ensiform or xiphoid appendix') The xiphoid process is the smallest of the three
pieces: it is thin and elongated, cartilaginous in structure in youth, but more or less ossified at its upper part in the
adult.
Surfaces
Its anterior surface affords attachment on either side to the anterior costoxiphoid ligament and a small part of the Rectus
abdominis; its posterior surface, to the posterior costoxiphoid ligament and to some of the fibers of the diaphragm and
Transversus thoracis, its lateral borders, to the aponeuroses of the abdominal muscles. Above, it articulates with the lower end
of the body, and on the front of each superior angle presents a facet for part of the cartilage of the seventh rib; below, by its
pointed extremity, it gives attachment to the linea alba. The xiphoid process varies much in form; it may be broad and thin,
pointed, bifid, perforated, curved, or deflected considerably to one or other side.
Structure
The sternum is composed of highly vascular cancellous tissue, covered by a thin layer of compact bone which is thickest in the
manubrium between the articular facets for the clavicles.
Ossification
The sternum originally consists of two cartilaginous bars, situated one on either side of the median plane and connected with
the cartilages of the upper nine ribs of its own side. These two bars fuse with each other along the middle line to form the
cartilaginous sternum which is ossified from six centers: one for the manubrium, four for the body, and one for the
xiphoid process [Fig. 4]. The ossific centers appear in the intervals between the articular depressions for the costal
cartilages, in the following order: in the manubrium and first piece of the body, during the sixth month; in the second and third
pieces of the body, during the seventh month of fetal life; in its fourth piece, during the first year after birth; and in the
xiphoid process, between the fifth and eighteenth years. The centers make their appearance at the upper parts of the segments,
and proceed gradually downward. 20 To these may be added the occasional existence of two small episternal centers, which make
their appearance one on either side of the jugular notch; they are probably vestiges of the episternal bone of the monotremata
and lizards. Occasionally some of the segments are formed from more than one center, the number and position of which vary [Fig.
6]. Thus, the first piece may have two, three, or even six centers. When two are present, they are generally situated one above
the other, the upper being the larger; the second piece has seldom more than one; the third, fourth, and fifth pieces are often
formed from two centers placed laterally, the irregular union of which explains the rare occurrence of the sternal foramen [Fig.
7], or of the vertical fissure which occasionally intersects this part of the bone constituting the malformation known as
fissura sterni; these conditions are further explained by the manner in which the cartilaginous sternum is formed. More
rarely still the upper end of the sternum may be divided by a fissure. Union of the various centers of the body begins about
puberty, and proceeds from below upward [Fig. 5]; by the age of twenty-five they are all united. The xiphoid process may become
joined to the body before the age of thirty, but this occurs more frequently after forty; on the other hand, it sometimes remains
ununited in old age. In advanced life the manubrium is occasionally joined to the body by bone. When this takes place, however,
the bony tissue is generally only superficial, the central portion of the intervening cartilage remaining unossified.
Figure 4 : Ossification of the sternum. |
Figure 5 |
Figure 6 : Peculiarities. |
Figure 7 |
Articulations
The sternum articulates on either side with the clavicle and upper seven costal cartilages.
This article is based on an entry from the 1918 edition of Gray's
Anatomy, which is in the public domain. As such, some of the
information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice
when it is no longer relevant.
See also
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