72 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



is 12 inches (31.5 cm.). The upper border of the sternum is 

 opposite the lower edge of the second dorsal (Henle), and the 

 lower border opposite the tenth dorsal. The sides slope out 

 to the ninth rib. The upper aperture is contracted and reni- 

 form, and sloped downward; the lower is irregular, and its 

 margin ascends on each side from the tenth rib to the ensiform, 

 forming the subcostal angle. The intercostal spaces are wider 

 above than below. 



The skeleton of the thorax comprises the dorsal vertebra, 

 the sternum, ribs, and costal cartilages. 



The dorsal vertebrae have been described. 



The Sternum 



The sternum is an azygos bone in the median line at the 

 front of the chest. It has attached the clavicles and seven 

 upper costal cartilages. It originally consisted of six segments, 

 and is likened to a sword. The upper segment remains dis- 

 tinct as the manubrium, or handle; the next four fuse into the 

 body, or gladiolus (little sword); the sixth portion is the ensi- 

 form or xiphoid process (sword-like). 



The sternum is flattened from before backward, and curved, 

 with a slight convexity, to the front. It is broad above, narrower 

 where the two upper segments meet, then broad again, and 

 narrow at the ensiform. 



The manubrium is the thickest part. It presents two sur- 

 faces, an anterior and a posterior, which are roughly quadri- 

 lateral. The anterior surface is convex from side to side, and 

 concave from above downward. It gives attachment, on each 

 side, to a portion of the pectoralis major aponeurosis, and the 

 sternal portion of the sternocleidomastoid muscle. The pos- 

 terior surface is concave and smooth. It gives attachment 

 to the sternohyoid and sternothyroid muscles of each side. 



Superiorly are three deep notches; the middle one is the 

 semilunar or inter clamcular notch; the lateral ones look up, 

 back, and out for articulation with the clavicles. Below the 

 lateral notches on either, side is a rough triangular surface 

 for union with the first costal cartilage; next is a sloping con- 

 cave surface; and at the lower angle a half-notch for the second 

 rib. The junction of the manubrium with the gladiolus is 



