The Ribs. 125 



seven. The transverse processes are vestigial in all except the third. 

 At the end of the column the segments are reduced to slender 

 cylinders of bone representing the vertebral bodies* 



THE RIBS. 



The ribs (costae) are twelve in number on either side. Each 

 is composed of a dorsal portion, the costal bone (os costale), or 

 bone-rib, and a ventral portion, the costal cartilage (cartilago 

 costalis) (Fig. 58). From their attachment on the vertebral 

 column the bone-ribs are directed outward, downward, and back- 

 ward. The costal cartilages are directed for the most part inward, 

 dowrnward, and forward. The first costal cartilage forms a pro- 

 nounced angle with the corresponding bone-rib. In the succeeding 

 ribs the angle is gradually replaced by a broad curve. 



Ribs are classified as true or sternal ribs (costae verae), and 

 false or asternal ribs (costae spuriae). The former — comprising 

 the anterior seven — are those directly attached to the sternum. 

 The latter — ^comprising the posterior five — are either indirectly 

 attached, or unattached. The unattached ribs are designated as 

 floating ribs. 



Generally speaking, the bone-ribs are cylindrical ; but the anterior 

 five or six are more or less flattened, with their main surfaces 

 respectively medial and lateral. The compression is most marked 

 in old animals. The first rib is extremely short. The succeeding 

 ribs increase in length to the sixth, and then decrease to the twelfth. 

 The arch formed by each rib has its greatest convexity, or angle, 

 at some point toward the dorsal surface. Passing backward, the 

 point of greatest convexity changes from a medial to a lateral 

 position. This, together with the elongation of the more posterior 

 ribs, results in an enormous increase in the posterior extent of the 

 thoracic cavity. 



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 



