The sternum is a long flat bone, evidently composed of several fused segments, that lies in the front wall of the chest, having the true costal cartilages attached to its sides (Fig. 47).
The bone consists, as already mentioned, of several parts, of which the first piece is the largest and is termed the manubrium, presternum, or episternum. The succeeding pieces that carry the remaining cartilages are taken together as constituting the body of the bone, mesoslernum or gladiolus.
The last piece is an irregular prolongation of the bone, usually bifid at its extremity and lying below and between the seventh cartilages, and is termed the xiphisternum, ensiform process, xiphoid cartilage, or metastemum*
The manubrium is the thickest and strongest portion. It articulates with the next piece by a layer of fibro-cartilage that does not ossify until late in life, and the two pieces do not lie quite in the same plane, but form an angle, so that the line of their junction is prominent, and is known as the angle of the sternum, or angle of Louis. Below this the body is composed of four segments, frequently termed sternebrce, and, although these segments are firmly united in the full-grown bone, the lines of junction are usually apparent as three transverse ridges on the front aspect of the body. They are not usually visible behind, though as a rule some indication of the division is to be found near the margins. Each sterncbra is flat, or very slightly concave, on its front aspect, and flattened behind. The sternum as a whole, however, shows a slight forward convexity. The metastemum, partly bone and partly cartilaginous, is set in a plane deeper than that of the body, but its lower end may project forward : its upper part may be partly hidden by the cartilages of the seventh ribs.
* The older names, manubrium, gladiolus and xiphoid, were given with reference to the fancied resemblance to a gladiator's sword, viz., handle, blade, and point. The other terms are better, in that tbey refer to the situation of the parts, although the term " episternum " is open to some objection on grounds of comparative anatomy.
The bone is broadest across the manubrium, and then opposite the junction of the fifth cartilages with it. It is narrow at the angle and still more so at the junction with the xiphoid.
The top border of the manubrium presents a central notch, the suprasternal, interclavicular, or jugular notch ; outside this is an articular facet for the clavicle on the upper lateral angle of the bone : the facet is concave from within out, slightly convex from before backward, and looks upwards and outwards and slightly backwards. The first rib cartilage reaches the bone, just below this facet, on the upper part of the side margin, and the second cartilage is connected with it opposite the angle, partly attached to the manubrium and partly to the first segment of the body. Thus the manubrium has one and a half cartilages connected with it on each side. On the body the third, fourth, and fifth cartilages are attached respectively opposite the lines between the segments as in the figure (Fig. 47), the sixth on the side of the last segment, and the seventh partly on this and partly on the xiphoid. In the figure the eighth cartilage on one side also reaches the xiphoid, an occurrence occasionally met with.
Thus each sternebra bears two half facets on each side for the cartilages : in the case of the second cartilage the articular cavity on the sternebra is separated from that on the manubrium by an interarticular ligament attached to the cartilage in the angle, making a double chondro-sternal articulation, and a corresponding condition may obtain in the third and fourth joints, but frequently in these, and usually in the remainder, the interarticular ligament is increased so that it obliterates one or both of the smaU cavities.
It may be noticed in this connection that there is usually some asymmetry of the bone, most apparent in the levels of its lateral articulations.
The manubrium, on its front surface, shows two ill-defined lateral hollows below the clavicular level, from which the Pectoralis major muscles arise. The vertical shghtly raised area between these hollows forms a T-shaped ridge with a transverse thickening that extends between the clavicular facets. Examination of the back of the bone shows a corresponding transverse elevation, and thus the thickest part of the manubrium, and indeed of the whole bone, is situated between the inner ends of the clavicles, evidently to withstand the pressure of these bones and shocks transmitted through them.
On the front ridge, running down and in from the facet, is a rough area on each side for the attachment of the sternal tendon of the Sterno-mastoid. Above and between these the bone is hollowed, covered by subcutaneous tissues and forming the floor of the supra-sternal hollow, and on the top margin is a roughness for the attachment of the interclavicular ligament.
The Pectorahs major meets its fellow in the middle line a variable distance above the angle ; its area of origin then takes the front surface of the mesosternum to the middle hne, as far as the level of the fifth cartilages, where the area narrows towards the sixth cartilage : this narrowing may commence one segment higher. All this origin of the muscle is by fleshy fibres, so that no corresponding rough marking occurs on the bone ; but the bare surface on the bone seen in the drawing below the areas for the muscles is really covered by a thick layer of interlacing tendinous fibres directly continuous with the Pectorals (see also Fig. 39), and these fibres, decussating in the centre, stream down over the end of the bone on to the surface of the xiphoid, where they are blended with the aponeurotic abdominal layers of the linea alba : these aponeuroses are the sheath of the Rectus, and some fibres of this muscle also reach the xiphoid.