The sternum, (os pectoris: xiphoides ; French : le sternum) is situated in the median line, at the fore part of the thorax: it is flat and narrow, but not of equal width in its entire extent, being broad at its upper part, then narrowed somewhat, after which it widens a little; finally it becomes compressed and narrow where it joins the ensiform cartilage. Its direction is oblique from above downwards and forwards; and the inclination forwards, together with the curve backward in the dorsal part of the vertebral column, causes a considerable increase in the anteroposterior diameter of the thorax. We have to consider successively its surfaces, extremities, and borders.
The anterior surface, slightly convex, and subjacent to the skin gives attachment to the aponeurosis of the pectoralis major and to the sternomastoid muscles and is marked by four transverse lines, indicating its original division into five pieces. The union between the first and second of these pieces (corresponding with the insertion of the second costal cartilages) is frequently cartilaginous even in adult age.
The posterior surface, somewhat concave, looks backward, towards the cavity of the thorax, and gives attachment, superiorly, to the sternohyoideus and sternothyroideus muscles; inferiorly, to the triangularis sterni. Along the middle line, it corresponds with the interval left by the divergence of the two pleurae (anterior mediastinum).
The borders are thick and marked on each side by seven angular depressions for the reception of the cartilages of the true ribs, which give them a notched or serrated appearance.
The superior extremity, broad and thick, is slightly excavated from side to side and presents at each corner a depression for the reception of the sternal end of the clavicle.
The sternum, in early infancy, is divided into several pieces, but in adult age two only remain distinct. These two pieces, with the ensiform appendage, at one time, received names derived from an imputed likeness of the whole to a sword; but the last-mentioned part now alone retains the designation grounded on this circumstance. *
Manubrium
The first division of the sternum 1 (manubrium or handle) is broader and thicker than the other; its form is nearly square; its lateral margins, thin and oblique, present each an oblong depression, which receives the cartilage of the first rib; and at each inferior angle may be observed an articular half notch, which articulates with the second rib. The superior border is hollowed, and hence the names incisure semilunaris or furcula, which have been applied to it. At the angles, which bound it are the fossae, which articulate with the clavicles, as has been already stated. The inferior border is straight and united to the upper extremity of the second piece.
The body
The second piece, much longer than the first, is marked on its anterior surface by some transverse lines, which indicate its original division into separate portions. Both surfaces are nearly flat.
The borders
The upper border is narrow, corresponding in breadth with the termination of the first bone, with which it is connected by cartilage.
The lateral margins present five notches for the reception of each of the cartilages of the five lower true ribs, and a half notch superiorly, which, with a similar depression in the first piece, forms a cavity for the second costal cartilage. The five inferior notches approach one another more closely in proportion as they are situated lower down, and part of the last is occasionally made up by the ensiform cartilage.
If the sternum is examined in several adult skeletons, it will be found to differ in form, — i. e. in the length of its parts, as well as in its breadth at given points; — but these differences are very various, and are not so considerable as to require detailed notice. Other peculiarities, less frequently met with, and of more importance, — such as divisions running through the bone, and perforations of its substance, — will be treated of more conveniently in the account to be given of the ossification; for there the manner of their production can be explained by reference to the process of growth.
The inferior extremity of the sternum
The inferior extremity of the sternum, thin and elongated, gives attachment to a cartilaginous appendix, 3 called the ensiform or xiphoid (a sword) cartilage, which in most cases remains in the cartilaginous state until an advanced period of life. Its form varies considerably in different individuals: and it is sometimes bent forwards, sometimes in the opposite direction, and sometimes pierced by a hole at its center. It gives attachment to the aponeurosis of the abdominal muscles.
Vesalius, while stating that others regarded the sternum and ensiform cartilage as resembling a sword, prefers to compare the pieces of the sternum to the handle only of that weapon, adding, in support of the view, the curious reason, thus expressed, "Secundum autem os, illi parti congruit, quam manus tota intiis complectitur, in qua sinus costarum carlilaginibus parati, eum preestant usum, quem in gladiis ex manubrij asperitate queerimus, quoties intortis nodosisq' funiculis, aut scabra piscis cute, illnd obduci curamus."
Articulations
The sternum articulates by its sides with the cartilages of the true ribs, — by its upper angles with the clavicles, and by the lower end, it is connected to the ensiform cartilage.
When sawed across, this bone presents a considerable quantity of loose spongy texture in its interior, with a very thin lamella of compact tissue on its outside; — hence it is very light.
Ossification
As far as the middle of fetal life, or a little later, the sternum is altogether cartilage. After that time the ossification begins with the formation of osseous granules in the middle of the intervals between the points at which the cartilages of the ribs are connected. There are five of these for the sternum exclusive of the ensiform appendage, and they form as many pieces. The process of ossification makes its appearance in the first piece between the fifth and sixth months, and soon following in the second and the third, it reaches the fourth at the end of fetal life. The osseous center of the last (fifth) varies considerably in the time of its appearance. It may be found soon after birth, and may not be visible for a considerable time (one or two years) after that period.
In many cases, one or more of the divisions of the sternum are formed from more nuclei than one, and there are peculiarities with respect to the number and position of these additional granules which require notice.
The first piece has often two points of ossification, placed usually one above the other; and it has been found to possess three. A number, which I believe to be very unusual (six), is contained in the preparation represented in figure d, 1'. The second has not often more than a single granule, but the third, fourth, and fifth divisions are frequently formed from two nuclei, which are placed laterally with respect to one another, — not vertically, as occurs in the first piece.
The presence of two points of ossification having the relative position mentioned, accounts for the vertical division sometimes found to run through one or more of the sternal pieces; and the occurrence of a hole, of various size, occasionally met within the middle of the sternum, is explained by reference to the same peculiarity in the manner of growth. Thus: — in the ordinary course the ossification extending uninterruptedly inwards from their nuclei the lateral parts meet and join to form a single piece before junction takes place with the piece immediately above or below. But supposing the formation of bone to cease when the parts are close together, the division which in all such cases exists for a time will become permanent, and if the growth should cease sooner, a larger interspace (a hole) will be the result (fig. 80, e). Fun her, if the interruption to the progress of ossification should occur at the point where the lateral parts of two sternal pieces would meet, the hole is likely to have considerable size, for it may then result from an " arrest of the development" proceeding from four centers, — each contributing a part.
The five pieces of the sternum constructed in the manner above detailed begin to join at the lower end of the bone. The fifth piece is joined to the fourth soon after puberty, the fourth and the third are united between twenty and twenty-five years of age, and the body of the sternum is usually not completed by the junction of the third piece to the second before thirty-five or forty years. Lastly, the first division does not, in general, join with the rest of the sternum at any period; but should its union happen to take place, it is only to be met with in old age.
To the centers of ossification here described M. Brechet* has added two small epi-sternal granules, whose position is sufficiently shown in the indication of them given in figure e. * * They occur only at rather advanced periods of life, but they do not appear to be constant.
The ensiform appendage begins to ossify some years after birth, — the time in different cases varying, according to the observation of Beclard, between two years and fifteen or eighteen. The ossification proceeds from a single center situated at the upper part, and from this, it gradually extends downwards, but in most instances, a portion remains cartilaginous even in very advanced age.