In anatomical language it is said, the ster num presents two surfaces, two borders, and two extremities.
Of the anterior or cutaneous surface of the is subcutaneous, slightly con vex and affords attachment to the aponeurosis of the pectoralis major and the sterno-cleido mastoid muscles. It presents three or four trans verse projecting lines, which are traces of the original division of the bone into five pieces. The union between the 1st and 2nd pieces cor responds to the insertion of the 2nd costal car tilage, and is frequently cartilaginous even in the adult age. The line which marks the union of the first two pieces of the bone is the most remarkable : it causes a projection of variable size in different individuals, which has been sometimes mistaken for a fracture or exos tosis. At the lower part we sometimes find a foramen ; sometimes in place of the foramen there is a considerable aperture, to which much importance has been attached, as afford ing a proof of the primitive separation of the bone in the median line. (Fig. 660. d.) The existence of this opening explains how puru lent matter deposited behind the sternum may, in certain cases, make its way outwards without any absorption of the bone. This bone is covered by a strong interlacement of very numerous aponeurotic fibres.
Of the posterior (mediastinal or cardiac) sur face.—This is slightly concave, and parallel in direction to the anterior surface. The con cavity is directed downwards and backwards, towards the cavity of the thorax, and gives attachment superiorly to the sterno-hyoideus and sterno-thyroideus muscles, inferiorly to the triangularis stern.
Along the middle line, this concavity cor responds with the interval left by the diverg ence of the two pleurtu (anterior mediasti num).
In the young subject, transverse lines arc seen corresponding to those which occupy the anterior surface ; all of these, except the two between the first and second pieces of the bone, are effaced at a more advanced age. This surface is in relation with many organs contained in the chest, and especially the heart, in front of which the sternum forms a kind of shield. This is exemplified, as al ready noticed, in the frog, which is provided with a sternum, though it has no ribs. At the
lower part of the sternum are many nutritient foramina.
Of the borders of the Sternum. — These are thick and marked at each side by seven an gular depressions for the reception of the cartilages of the first seven ribs, which gives this bone a notched and serrated appear ance. These angular cavities are separated from each other by semilunar notches, which are longer above than below, where the faccttes closely approach each other. The uppermost of these seven cavities is shallow, triangular, and at an early age becomes ingrained with the cartilage of the first rib ; those which follow are deeper, angular, and situated at the ex tremities of each of the transverse lines. When examined in the dried specimen, they appear more angular and deeper in proportion to the youth of the subject.
Of the clavicular extrenzity. —This is slightly convex, and is the broadest and thickest part of the whole bone. It is slightly excavated from side to side, and presents at each corner a depression for the reception of the sternal end of the clavicle, which bears the name fourchette; this is surrounded with irregula rities for the insertion of ligaments. It fre quently happens that the two clavicular arti culations are not at the same height ; a fact which was noticed by Morgagni, and which Cruveilhier attributes to the unequal wearing of the two articular surfaces. We have once seen the clavicular articulation so low as to unite with the first costal cartilage.
Of the inferior extremity of the Sternum. This is formed by the ,ri,eboid appendix ; or ensiform cartilage, for it often remains cartilaginous to adult age. In length, shape, and direction, it presents nume rous varieties ; it is frequently bifid, some times pierced by a foramen, and is occasion ally bent forwards, or to one side, and in certain cases much depressed : its summit gives attachment to an aponeurotic structure, called the linea alba ; behind, it indirectly corresponds with the stomach, which rests upon it when the body is placed in a prone position.