THE STERNUM 121 



fourth, and fifth articular depressions. 1 It affords attachment on either side to 

 the sternal origin of the Pectoralis major. At the junction of the third and fourth 

 pieces of the body is occasionally seen an orifice, the sternal foramen, of varying 

 size and form. The posterior surface, slightly concave, is also marked by three 

 transverse lines, less distinct, however, than those in front; from its lower part, 

 on either side, the Transversus thoracis takes origin. 



Borders. The superior border is oval and articulates with the manubrium, the 

 junction of the two forming the sternal angle (angulus Ludovici" 1 ). The inferior 

 border is narrow, and articulates with the xiphoid process. Each lateral border 

 (Fig. 117), at its superior angle, has a small facet, which with a similar facet on 

 the manubrium, forms a cavity for the cartilage of the second rib; below this 

 are four angular depressions which receive the cartilages of the third, fourth, 

 fifth, and sixth ribs, while the inferior angle has a small facet, which, with a cor- 

 responding one on the xiphoid process, forms a notch for the cartilage of the seventh 

 rib. These articular depressions are separated by a series of curved interarticular 

 intervals, 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 lines 

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

 the lower animals, where the parts of the bone remain ununited longer than in 

 man. 



Xiphoid Process (processus xiphoideus; ensiform or xiphoid appendix). The 

 xiphoid process is the smallest of the three pieces: it is thin and elongated, 

 cartilaginous in structure in youth, but more or less ossified at its upper part in 

 the adult. 



Surfaces. Its anterior surface affords attachment on either side to the anterior 

 costoxiphoid ligament and a small part of the Rectus abdominis; its posterior sur- 

 face, to the posterior costoxiphoid ligament and to some of the fibers of the dia- 

 phragm and Transversus thoracis, its lateral borders, to the aponeuroses of the 

 abdominal muscles. Above, it articulates with the lower end of the body, and 

 on the front of each superior angle presents a facet for part of the cartilage of the 

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

 alba. The xiphoid process varies much in form; it may be broad and thin, pointed, 

 bifid, perforated, curved, or deflected considerably to one or otner side. 



Structure. The sternum is composed of highly vascular cancellous tissue, covered by a thin 

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

 clavicles. 



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

 side of the median plane and connected with the cartilages of the upper nine ribs of its own side. 

 These two bars fuse with each other along the middle line to form the cartilaginous sternum which 

 is ossified from six centers: one for the manubrium, four for the body, and one for the xiphoid 

 process (Fig. 118). The ossific centers appear in the intervals between the articular depressions 

 for the costal cartilages, in the following order: in the manubrium and first piece of the body, 

 during the sixth month; in the second and third pieces of the body, during the seventh month of 

 fetal life; in its fourth piece, during the first year after birth; and in the xiphoid process, between 

 the fifth and eighteenth years. The centers make their appearance at the upper parts of the seg- 

 ments, and proceed gradually downward. 3 To these may be added the occasional existence of 

 two small episternal centers, which make their appearance one on either side of the jugular notch; 

 they are probably vestiges of the episternal bone of the monotremata and lizards. Occasionally 

 some of the segments are formed from more than one center, the number and position of which 

 vary (Fig. 120). Thus, the first piece may have two, three, or even six centers. When two are 



1 Paterson (The Human Sternum, 1904), who examined 521 specimens, points out that these ridges are altogether 

 absent in 26.7 per cent. ; that in 69 per cent, a ridge exists opposite the third costal attachment; in 39 per cent, opposite 

 the fourth; and in 4 per cent, only, opposite the fifth. 



2 Named after the French surgeoa Antoine Louis, 1723-1792. The Latin name angulus Ludovici is not infrequently 

 mistranslated into English as "the angle of Ludwig. " 



3 Out of 141 sterna between the time of birth and the age of sixteen years, Paterson (op. cit.) found the fourth or 

 lowest center for the body present only in thirty-eight cases i. e., 26.9 per cent. 



