BONES. 



often piered in the centre ; it partly supports the seventh costal 

 cartilage. [96] 



Ribs. There are twelve pairs. The costal cartilages of the 

 upper seven (true ribs) join the sternum; those of the lower 

 five (false ribs) do not. Of the latter, the eighth, ninth, and 

 tenth costal cartilages join the cartilage of the seventh; the 

 anterior ends of the lower two (floating ribs) are free. A typical 

 rib has a head, neck, tubercle, and shaft, with its angle. The 

 head is expanded; a horizontal ridge, for the interarticular 

 ligament, divides its wedge-shaped inner end into two facets, 

 upper and lower, for the bodies of the vertebrae. The neck is 

 a constriction next the head, smooth in front and rough be- 

 hind, with ridges on its upper border as a rule. The tubercle 

 lies at the junction of the neck and shaft posteriorly and has 

 two parts; the non-articular part lies externally and above, 

 and is rough; the articular part is rounded, or oval, and di- 

 rected downward, backward, and slightly inward. [97] 



The shaft is flattened and curved, the upper ribs curv- 

 ing most acutely; the seventh and eighth are the longest. 

 Toward the back of each rib the curve is sharpest and here 

 (the angle} a ridge crosses the outer surface obliquely. The 

 distance between the angle and tubercle is greatest on the 

 eighth rib; it diminishes rapidly above but only slightly be- 

 low. The shaft also twists, so that the anterior end turns 

 downward and the posterior upward. The outer surface is 

 smooth and directed upward in the upper rib; upward and 

 outward in the ribs next below ; outward in the middle ribs ; 

 and outward and slightly downward in the lower ones. In the 

 middle ribs an oblique ridge (anterior angle) may cross the 

 outer surface anteriorly. The internal surface is conversely 

 directed. The upper border is thick behind and thinner in 

 front. The lower border is grooved behind, at the expense of 

 the inner surface, and has a sharp and overhanging external 

 margin; this (subcostal) groove fades anteriorly and its lips 

 coalesce to a rounded edge. [98] 



[109] 



