THE STERNUM, OR BREAST BONE 159 



similar facet on the upper angle of the middle portion of the bone, forms a notch 

 (incisura costalis II} for the reception of the costal cartilage of the second rib. 

 These articular surfaces are separated by a narrow, curved edge, which slopes 

 from above downward and inward. 



Second Piece. The second piece, or gladiolus, considerably longer, narrower, 

 and thinner than the first piece, is broader below than above. 



Surfaces. Its anterior surface (planum sternale) is nearly flat, directed upward 

 and forward, and marked by three transverse lines which cross the bone opposite 

 the third, fourth, and fifth articular depressions. These lines are produced by the 

 union of the four separate pieces of which this part of the bone consists at an early 

 period of life. At the junction of the third and fourth pieces is occasionally seen 

 an orifice, the sternal foramen; it varies in size and form in different individuals 

 and pierces the bone from before backward. This surface affords attachment 

 on each side to the sternal origin of the Pectoralis major. The posterior surface, 

 slightly concave, is also marked by three transverse lines, but they are less dis- 

 tinct than those in front; this surface affords attachment below, on each side, 

 to the Triangularis sterni muscle, and occasionally presents the posterior opening 

 of the sternal foramen. 



Borders. The superior border presents an oval surface for articulation with the 

 manubrium. The inferior border is narrow, and articulates with the ensiform 

 appendix. Each lateral border presents, at each superior angle, a small facet, 

 which, with a similar facet on the manubrium, forms a cavity for the cartilage of 

 the second rib; the four succeeding angular depressions receive the cartilages of 

 the third, fourth, fifth, and sixth ribs; while each inferior angle presents a small 

 facet, which, with a corresponding one on the ensiform appendix, forms a notch 

 for the cartilage of the seventh rib. These articular depressions are known as 

 incisurae costales. They are separated by a series of curved interarticular inter- 

 vals, which diminish in length from above downward, and correspond to the 

 intercostal spaces. Most of the cartilages belonging to the true ribs, as will be 

 seen from the foregoing description, articulate with the sternum at the line of 

 junction of two of its primitive component segments. This is well seen in many 

 of the lower animals, where the separate parts of the bone remain ununited 

 longer than in man. In this respect a striking analogy exists between the mode 

 of connection of the ribs with the vertebral column and the connection of the 

 costal cartilages with the sternum. 



Third Piece. The third piece, the ensiform or xiphoid appendix, is the smallest 

 of the three; it is thin and elongated in form, cartilaginous in structure in youth, 

 but more or less ossified at the upper part in the adult. 



Surfaces. Its anterior surface affords attachment to the chondroxiphoid liga- 

 ment; its posterior surface, to some of the fibres of the Diaphragm and Triangu- 

 laris sterni muscles; its lateral borders, to the aponeurosis of the abdominal muscles. 

 Above it articulates with the lower end of the gladiolus, and at each superior 

 angle presents a facet (incisura costalis VII), for the lower half of the cartilage 

 of the seventh rib; below, by its pointed extremity, it gives attachment to the linea 

 alba. This portion of the sternum varies much in appearance, being some- 

 times pointed, broad, and thin, sometimes bifid or perforated by a circular open- 

 ing, occasionally curved or deflected considerably to one or the other side. 



Structure. The bone is composed of delicate cancellous structure, covered by a thin 

 layer of compact tissue, which is thickest in the manubrium between the articular facets for 

 the clavicles. 



Development. The cartilaginous sternum originally consists of two bars, situated one on 

 either side of the mesal plane and connected with the rib cartilages of its own side. It is usual 

 for the eighth cartilage to lose its attachment to the sternum and become attached to the seventh 

 cartilage. The sternal end of the ninth cartilage divides longitudinally, the mesal part remains 



