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, 
downward, 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 
