86 HANDBOOK OF ANATOMY 



The Ribs. 



The Ribs (costae) are twenty -four in number, twelve on each 

 side. They articulate posteriorly with the dorsal vertebrae, 

 and anteriorly with the sternum and one another (except the 

 eleventh and twelth, which are free at their anterior ends), 

 thus forming the lateral walls of the thorax. 



A rib is a long, narrow, flat bone, consisting of a head and 

 neck attached to the shaft. The head is somewhat expanded, 

 and has on its inner surface a facet for articulation with the 

 Vertebrae, the lower half of which corresponds with the facet 

 on the upper margin of the body of the vertebra corresponding 

 with it, so that each rib articulates with the body of the vertebra 

 with which it is in numerical sequence, and with the body of the 

 one above. Thus, the seventh rib articulates with the upper 

 margin of the body of the seventh vertebra and the lower margin 

 of the body of the sixth. Below, the head is constricted to form 

 a neck, and where it joins the shaft there is a tubercle on the 

 posterior surface. The tubercle has a facet to articulate with 

 the transverse process of the vertebra numerically correspond- 

 ing with it. The shaft is long, flat, and narrow. It is convex 

 laterally, but the curve is much sharper posteriorly than 

 anteriorly, and where the curve is sharpest there is a rough, 

 oblique ridge on the external surface ; this is called the angle. 

 On the first rib the tubercle and angle coincide. They gradually 

 separate, being farthest apart on the eighth rib, and approach- 

 ing one another again on the lower ones. The shaft of the rib 

 is also twisted on itself, so that, if laid on a flat surface, one end 

 is always raised up. This is most marked in the middle ones, and 

 not found at all in the first, second, and twelfth. The upper 

 border is thick and rounded, the lower one thin and sharp, and 

 on the inner surface of the lower border is a groove for vessels 

 and nerves. The anterior end of the shaft is slightly expanded 

 and hollowed out for the attachment of the costal cartilage, 

 by means of which the ribs articulate with the sternum or one 

 another. 



The first rib is much smaller than the others, quite flat, and 

 forms nearly a semicircle. On its upper surface, about midway, 



