THORAX. 133 



more marked as it is more posterior, the groove of 

 the external face becomes less marked as th& rib is 

 narrower. 



First rib — Has no groove on the external face, no 

 fissure between the facets of the head, shows large 

 muscular impressions on the external face, and an ar- 

 ticular facet on the inferior extremity of the internal 

 face. 



Last rib — Has no groove externally, the facet of 

 the tuberosity and the posterior facet of the head 

 are united. 



Costal Cartilages.— Those of the eight first ribs rest 

 on the sternum by a facet after articulating above 

 with the inferior extremity of the rib. Those of the false 

 ribs are united with their corresponding ribs by their 

 inferior extremity, which terminates in a point, and 

 rests directly upon each other. 



Sternum.— Situation: under the thorax. Division: 

 Three faces, three borders, two extremities. The su- 

 perior face is concave and triangular ; the lateral face 

 is divided into a superior portion which shows diar- 

 throdial cavities and an inferior portion for muscular 

 attachments. The lateral border separates the latter 

 from the superior face ; the inferior border is convex, 

 thin and resembles tlie keel of a ship. The anterior 

 extremity forms tlie trachelian prolongation ; the pos- 



