THE STERNUM OR BREASTBONE. 25 



the incisura semilunaris, or interclavicular notch ; the lateral ones form two depressed 

 articular surfaces, directed upwards, outwards and backwards, for articulation with 

 the clavicles, and called the clavicular notches. Each lateral border presents supe- 

 riorly, close to the clavicular notch, a rough triangular surface, which unites with 

 the cartilage of the first rib. Below this the bone slopes inwards, and at its inferior 

 angle presents a small surface, which with a similar one on the body forms a notch 

 for the cartilage of the second rib. The lower margin is straight and united by 

 cartilage to the upper margin of the body. 



The body (gladiolus, mesosternum) is marked on its anterior surface by three 

 slight transverse elevations at the lines of junction of its four component parts. Its 

 posterior surface is comparatively smooth. Each lateral margin presents five notches 

 for the reception of costal cartilages, and a small surface above, which, with the 

 similar depression on the manubrium, forms the notch for the second costal cartilage. 

 The notches for the third, fourth, and fifth costal cartilages are opposite the lines of 

 junction of the four segments of the body of the sternum ; those of the sixth and 

 seventh are placed close together on the sides of the inferior segment, that for the 

 seventh being completed by the ensiform process. 



The ensiform or xiphoid process (metasternum, xiphisternum) is a thin 

 spatula-like process projecting downwards between the cartilages of the seventh 

 ribs. It is subject to frequent varieties of form, being sometimes bent forwards, 

 sometimes backwards, often forked, and occasionally perforated. 



The sternum is subcutaneous in the middle line, forming the floor of the sternal 

 groove between the pectoral muscles, which cover the lateral portions of the anterior 

 surface. The upper end is marked by the deep suprasternal notch; and the ensiform 

 process lies at the bottom of the infrasternal depression, the latter being due to the 

 prominence of the body and the seventh costal cartilages beyond the surface of the 

 ensiform process. 



The length of the sternum and the proportions of its parts differ somewhat in the two 

 sexes. In the male the body is as a rule slightly more than twice as long as the manubrium ; 

 while in the female the whole bone is relatively shorter, and the body is usually less than 

 twice the length of the upper segment. Individual variations are, however, frequent and great. 

 (M. Strauch, Diss., Dorpat, 1881 ; T. Dwight, Journ. Anat., xv, 327, and xxiv, 527 ; F. Peter- 

 moller, Diss., Kiel, 1890.) 



Varieties. The sternum is subject to many varieties of form. It is not unfrequently 

 much shorter than usual, and indented at its lower part, as occurs especially from the pressure 

 of the cobbler's last. Occasionally the lower part of the body is perforated by the so-called 

 sternal foramen (fig. 30, E) ; and in rare cases the sternum has been found divided to a 

 greater or less extent, constituting the malformation of fissura sterni, and connected in some 

 instances with ectopia cordis. 



Two small nodules of bone, ossa suprastcrnalia, have been found in some rare cases at the 

 upper border, close to the clavicular notches, united by cartilage and ligament to the sternum. 

 Their position is indicated by the asterisks (**) in figure 30, E. They appear to be vestiges of 

 the episternal bone of monotremata and lizards, the lateral parts of which are represented 

 normally in the interarticular fibro-cartilages of the stemo-clavicular articulations. 



THE RIBS. 



The ribs (costce), twelve in number on each side, constitute a series of arched 

 and highly elastic bones, which extend outwards and forwards from the vertebral 

 column, and form the lateral walls of the thorax. Their anterior extremities give 

 attachment to cartilaginous prolongations the costal cartilages, the first seven pairs 

 of which pass inwards to the sternum. On this account the first seven pairs of ribs 

 are called sternal, also true, and the remaining five pairs asternal or false ribs. Of 

 these asternal ribs, each of the upper three has its cartilage attached along its supsrior 

 border to the cartilage of the rib above it ; while the two last are entirely free from 

 such attachment, and are thence called floating ribs. 



