74 Anatomy oi^ the Rabbit. 



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 in the last four, the tubercle bears a 

 sharp, dorsally-directed process for muscular attachment. The slender 

 portion of the rib intervening between the head and tubercle is the neck 

 (collum costae), the remaining larger portion being distinguished as 

 the body of the rib (corpus costae). 



The bony thorax is formed by the ribs and the sternum with the 

 assistance of the thoracic vertebrae. It encloses a large space, the 

 thoracic cavity (cavum thoracis). The latter is conical in shape, with 

 the apex directed forward. The dorsoventral diameter of the cavity 

 is considerably greater than the transverse diameter. Apart from the 

 intercostal spaces, the cavity is open at two points: anteriorly, the first 

 thoracic vertebra, the first rib, and the manubrium sterni together 

 enclose a small opening, the superior thoracic aperture (apertura thoracis 

 superior) ; posteriorly, the seventh and succeeding ribs, together with 

 the posterior thoracic vertebrae and the xiphoid process of the sternum, 

 enclose a much larger opening, the inferior thoracic aperture (apertura 

 thoracis inferior). In the natural condition it is largely closed by the 

 diaphragm. The curved boundary formed by the ribs in this region 

 is the costal arch (arcus costarum), the angle formed at the point of 

 attachment of the xiphoid process being the infrasternal angle (angulus 

 infrasternalis). 



Through their articulations with the vertebral column, and the nature 

 of the costal cartilages, the ribs are capable of being moved, or rotated, 

 forward. The movement results in an increase of the extent of the 

 thoracic cavity, and is of importance in respiration. 



THE STERNUM. 



The sternum consists of a linear series of six segments, the sternebrae. 

 The first segment is the manubrium sterni. It is about twice the length 

 of the middle segments. It is somewhat triangular in section, two of 

 its surfaces being ventrolateral, the third dorsal and directed toward 

 the thoracic cavity. To its anterior tip is attached the sternoclavicular 

 ligament, by which the greatly reduced clavicle is united with the 

 sternum. 



