The
sternumThe sternum is a long flat bone shaped like a capital 'T' located in the center of the thorax...
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 processThe xiphoid process, also known as the xiphisternum, is a small cartilaginous extension to the lower part of the sternum which is usually ossified in the adult human. By age 15 to 29, the xiphoid usually fuses to the body of the sternum with a fibrous joint. Unlike the synovial articulation of...
[Fig.
The
sternumThe sternum is a long flat bone shaped like a capital 'T' located in the center of the thorax...
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 processThe xiphoid process, also known as the xiphisternum, is a small cartilaginous extension to the lower part of the sternum which is usually ossified in the adult human. By age 15 to 29, the xiphoid usually fuses to the body of the sternum with a fibrous joint. Unlike the synovial articulation of...
[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 of fetal life; 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. 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
pubertyPuberty is the process of physical changes by which a child's body becomes an adult body capable of reproduction. Puberty is initiated by hormone signals from the brain to the gonads . In response, the gonads produce a variety of hormones that stimulate the growth, function, or transformation of...
, 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.
External Links: Gray (1821–1865). Anatomy of the Human Body. 1918. The Sternum.