THE STERNUM. 93 
with of spatula-shape, bifid, circular, pierced in the centre, or twisted and deflected 
to one or other side, or turned forward. 
The sternum as a whole is broadest above where the first rib cartilages are 
attached. It becomes narrow opposite the second rib cartilages, but again expands 
until the level of the fifth rib cartilage is reached, below which it is rapidly 
reduced in width and ends below in the pointed xiphoid cartilage. Its position in 
the body is oblique from above downwards and forwards; its axis, if prolonged 
upwards, would touch the column opposite the third or fourth cervical vertebra. 
Though lable to changes in position by the rising and falling of the chest-wall, its 
upper extremity corresponds to the level of the lower border of the second dorsal 
vertebra, whilst the lower end of the xiphoid cartilage usually falls in line with the 
disc between the tenth and eleventh dorsal vertebree. 
In women the sternum is usually narrower and shorter than in men, and its position less 
oblique. 
Architecture.—It consists of large-celled spongy bone, which is highly vascular, and is con- 
tained between two layers of thin compact tissue. 
Ossification. — The cartilaginous sternum, developed from the fusion mesially 
of two cartilaginous bands uniting the anterior extremities of the cartilages of the 
first eight ribs, begins to ossify 
about the sixth month of fetal 
life. About this time a single 
centre appears in the manu- 
brium; at birth this is well 
developed. Secondary epiphyses 
have been decsribed in connexion 
with the clavicular facets ; these 
do not unite with the rest of 
the manubrium till adult life 
is reached. The body formed 
by the fusion of four segments 
is ossified from independent cen- 
tres, either single or double, for 
each segment. These appear— 
the highest as early as the sixth 
month of intra-uterine life— 
in some cases even before the 
manubrium has begun to ossify 
(Lambertz), the lowest toward the 
end of full term. The common 
arrangement met with at birth Fic. 71.—OSSIFICATION OF THE STERNUM. 
Ric a single centre for the first, 1 this figure the second as well as the third segment of the body 
and double centres for each of Pan pee seidiniieret | ae Ail i: 
i 7 . . Tnic . 9pears abou a OF month, a. £ pear about th 
ee ee poe month ; aoe from 20 to 25. 3. Appear about sth or 9th month : 
5 , III. segment unites with II. about puberty ; IV. segment unites 
rather irregularly, and is liable with IIT. in early childhood. 4. Appears about 3rd year or later. 
to much variation. The fourth 
unites with the third segment in early childhood, the third with the second about 
puberty, whilst the fusion of the second with the first segment may not be complete till 
the twentieth or twenty-fifth year. 
The xiphi-sternum usually ossifies from a single centre, which may appear as early as 
the third year, though often very much later. The xiphi-sternum usually unites with the 
body about forty or fifty, and in exceptional cases osseous union between the body and 
manubrium may occur in advanced life. 
At birth. 
At 3 years. 
Variations.—The sternum is liable to considerable individual variations affecting its length 
and direction. The majority of bones are asymmetrical, displaying irregularities in the levels of 
the clavicular facets. The higher costal facets may be closer together on one, usually the right 
side, than the other, whilst the pre-mesosternal joint is often oblique, sloping somewhat to the 
right. According to Birmingham, these are the result of the strain thrown on the shoulder by 
pressure either directly apphed or through the pull of a weight carried in the hand. 
Sometimes the sternum articulates with eight rib cartilages. This may happen on one or 
both sides, but when unilateral, much more frequently on the right side—a condition by some 
associated with right-handedness. It is, however, more probably a persistence of the primitive 
condition of the cartilaginous sternum, in which each half is connected with the anterior 
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