THE RIBS 



165 



cr.c. 



O.C. 



The vertebral end, or head of the rib (capitulum costae), is 

 articulated with the body of the vertebra to which it belongs and 

 also, in the case of the first ten, with the vertebra immediately in 

 front. The articulation with a transverse process is marked by a 

 small smooth elevation, the costal tubercle (tuberculum costae). 

 It is present only in the first nine ribs.^ Except in the first rib and 

 in the last four, the tubercle bears a sharp, dorsally-directed process 

 for muscular attachment. The slender portion of the rib inter- 

 vening betw^een the head and tubercle is the neck (collum costae), 

 the remaining larger portion being distinguished as the body of 

 the rib (corpus costae). 



The bony thorax is formed by the ribs and the sternum with the 

 assistance of the thoracic vertebrae. It encloses a large space, the 

 thoracic cavity (cavum thoracis). The latter is conical in shape, 

 with the apex directed forward. 

 The dorsoventral diameter of the 

 cavity is considerably greater than 

 the transverse diameter. Apart 

 from the intercostal spaces, the 

 cavity is open at two points : anteri- 

 orly, the first thoracic vertebra, the 

 first rib, and the manubrium sterni 

 together enclose a small opening, 

 the superior thoracic aperture 

 (apertura thoracis superior) ; pos- 

 teriorly, the seventh and succeed- 

 ing ribs, together with the posterior 

 thoracic vertebrae and the xiphoid 

 process of the sternum, enclose a 

 much larger opening, the inferior 

 thoracic aperture (apertura tho- 

 racis inferior). In the natural con- 

 dition it is largely closed by the 

 diaphragm. The curved boundary 

 formed by the ribs in this region 

 is the costal arch (arcus costarum), 

 the angle formed at the point of 

 attachment of the xiphoid process 

 being the infrasternal angle (angulus infrasternalis). 



Fig. 84. The sternum and first rib, 

 ventral view: 1-7, the true ribs; 8, first 

 false rib; c.c, head of rib; cl.c, neck of 

 rib; cr., costal cartilage; cr.c, body of 

 rib; c.s., body of sternum; m.s., manu- 

 brium sterni; o.c, bone-rib; p.x., xiphoid 

 process; t.c, costal tubercle. 



