THE MBS AND STEENtfM 



229 



to the vertebrEB by two processes, one of which rests 

 against the transverse process and the other against the 

 centrum. At the front of the body the curved ends of 

 the first seven pairs end in cartilages which unite them 

 to the flat breastbone, or sternum. The eighth, ninth, 

 and tenth pairs end in similar cartilages, but these are 

 fused together into one common cartilage which is at- 

 tached to the lower 

 part of the breast- 

 bone. The eleventh 

 and twelfth pairs are 

 not attached, and are 

 called floating ribs. 



The sternum is a 

 flat, irregular bone 

 formed of three parts 

 in the adult. The 

 upper piece is broad, 

 and the collar bones 

 fit into notches in the 

 top. On the sides are 

 two other notches to 

 which the first pair 

 of ribs is 'attached. 

 The middle piece is 

 narrower, and to it 

 are attached the fourth, fifth, and sixth pairs of ribs. The 

 last piece has no rib attachments. The ribs are so 

 arranged as to permit the expansion and contraction of 

 the organs of the chest cavity and at the same time to 

 protect the inclosed organs. We shall consider their 

 action further in our study of respiration. 



Fig. 88 — Kibs and sternum. 



