TUB SKELETAL SYSTEM 29 



ceeding six pairs of ribs are attached between the seg- 

 ments. The seventh segment (xiphoid process) Ls not asso- 

 ciated with a pair of ribs. It terminates in a round plate 

 of cartilage. 



Exerolse VI. Draw and label the sternum. 



There are thirteen pairs of rihs. A rib contains a dor- 

 sal and a ventral segment. The anterior seven pairs are 

 the true ribs, for they are attached to the sternum. The 

 six posterior pairs are the false ribs. The eighth, ninth, 

 and tenth pairs attach to the posterior border of the sev- 

 enth, and by cartilage to the fifth sternebra. The three 

 posterior ribs are floating ribs, lacking ventral segments. 

 The ribs are bow-shaped in general, the angle, or region 

 of greatest convexity, being dorsal. Most of the ribs are 

 distinctly flattened anteroposteriorly. From the fifth to 

 the tenth pairs, inclusive, the distal end of the rib is 

 twisted, so that the broad surface is posterolaterad. Com- 

 pare the shapes of the different ribs. 



The fifth rib may be used to illustrate the articular sur- 

 faces. The capitulum cosiae (head) is a slight swelling at 

 the proximal end. Its circular medial surface articulates 

 with the demifacets of two adjacent vertebrae, as previ- 

 ously described. The tuberculum costae (tubercle) is a 

 dorsal enlargement two or three millimeters lateral to the 

 head. It articulates with the transverse process of the 

 vertebra, and is separated from the head by the collwm 

 costae {neck) of the rib. 



The thoracic cavity, surrounded on the sides by the ribs, 

 dorsally and ventrally by the backbone and sternum re- 

 spectively, resembles an ice cream cone with the apex cut 

 off. The cranial thoracic aperture is in front, the larger 

 caudal thoracic aperture behind. Both are circumscribed 

 by ribs, sternum, and backbone. The thoracic cavity in- 



