Bones of the Thorax. 83 



The sternum (breastbone) (see also figures 105 and 107) lies as an unpaired oblong 

 plate of bone in the median plane and helps to form the anterior wall of the thorax. It 

 extends in general somewhat obliquely from behind and above, forward and downward and 

 corresponds in position to the spine from the third to the ninth thoracic vertebra. Three 

 parts are distinguishable: manubrium sterni, corpus sterni and processus xiphoideus , the 

 parts being separated from one another by layers of cartilage which, however, in advanced age 

 may ossify and lead to fusion. 



The manubrium sterni is the broadest part, especially above; below it becomes 

 somewhat narrower; its anterior surface is slightly convex, its posterior, flat. At the upper 

 margin in the middle is a flat notch, incisura juyularis (0. T. presternal notch) ; near this, 

 at the upper angle on each side, is a somewhat deeper depression covered with cartilage, the 

 incisura clavicularis, for articulation with the facies articularis sternalis claviculae. 



The corpus steriii (0. T. gladiolus) is the longest portion; above and below it is 

 somewhat narrower than in the middle ; its anterior surface, planum sternate, and its posterior 

 surface are flat. The manubrium and corpus are somewhat movable upon one another where 

 they are united at the synchondrosis sternalis (see Fig. 226); at the point of junction, whether 

 the joint exists or even when it has become ossified, there is an angle, angulus sterni, the size 

 of which varies according to age, sex and individual; it changes during respiration. 



The processus xiphoideus (0. T. ensiform process) forms the lower extremity; it is 

 thin, longer than it is broad, very irregular in shape, often bifurcated or perforated by an 

 opening and remains for a long time wholly or partially cartilaginous. Only late in life does 

 it begin to become ossified inside and to fuse with the body. 



On each lateral surface of the manubrium and corpus are seven (sometimes eight, see 

 p. 78) notches, incisurae costales, for the upper seven (eight) ribs. The uppermost is rough, 

 the others are covered with cartilage. The uppermost notch is in the manubrium just below 

 the incisura clavicularis; the second at the point of union of manubrium and corpus, each 

 forming a half of it; the third on the corpus nearly in the middle of the whole length of 

 manubrium -j- corpus ; the fourth to the seventh (eighth) in the lower half, so that the inferior 

 notches are closer and closer together, the two lowest often being fused to one; the seventh 

 (and eighth) situated at the junction of the corpus with the processus xiphoideus may be 

 situated partially in front of the latter. 



In the female, other things being equal, the corpus sterni is shorter than in the male ; 

 accordingly, the female sternum is, on the whole, shorter and looks relatively broader and 

 plumper. 



