THE RIBS, THE STERNUM, AND THE THORAX 155 



the ball-and-socket joint between the head of the rib and the vertebral 

 bodies and the nature of the facet on the transverse process, whereof 

 the concavity is along a cephalo-caudal line, permit the rotation of the 

 vertebral end of the rib about its own axis. This rotation is effected 

 by muscles passing from the rib to the vertebral column, the other 

 ribs, the sternum, and even to the limbs. In the enlargement of the 

 thorax in inspiration the rotation of the rib is on the arc of a circle 

 toward the tail and ventrally. The movement, small at the vertebral 

 end, becomes greater at the sternal end, especially in the long caudal 

 ribs, which are more loosely joined to the vertebral column. Owing 

 to the curve in the rib and its oblique direction, ventrally and toward 

 the tail, the rotation of its vertebral end moves the sternal end toward 

 the head and ventrally, and thus increases the cephalo-caudal diameter 

 by moving the whole thorax toward the head, at the same time in- 

 creasing the dorso-ventral diameter of the thorax by bringing the rib 

 more nearly perpendicular. The rotation, finally, turns the convexity 

 of the rib, which especially in the caudal ribs faces toward the tail 

 and laterally, directly laterally, and thus increases the transverse 

 diameter of the thorax. As the costal cartilages bend medially, 

 ventrally, and toward the head, they oppose this rotation toward 

 the head and laterally of the ribs, and, when the muscles of inspira- 

 tion cease to act, return the ribs to their former position, producing 

 the expiration of the air with the minimum expenditure of muscular 

 energy. 



HUMAN RIBS. 



The skeleton of man has twelve pairs of ribs, whereof seven are 

 directly united to the sternum by their cartilages. In three of the 

 remaining five pairs the cartilages turn toward the head and join the 

 caudal borders of their immediate predecessors, and in the last two 

 pairs the ventral ends are free in the muscle of the abdominal wall. 

 Thus there are seven pairs of true ribs and five pairs of false ribs, 

 whereof two pairs are floating ribs. It will be remembered that the cat 







has nine pairs of true ribs and four pairs of false ribs, whereof one pair 

 only are floating ribs. 



A TYPICAL HUMAN RIB. 



On comparing the fifth rib of man with the corresponding rib of 

 the cat, the student will observe that it has a more decided dorso- 



