THE BONES. 



25 



13. The Walls of the Thorax. The cavity of the 

 chest contains the heart, lungs, and larger blood-vessels. It 

 is cone-shaped, with the apex at the top. The walls consist 

 of bone, muscle and elastic cartilage. Behind is the strong 

 spinal column. In front, extending from the neck to the pit 

 of the stoinachj^ |sjbhe_ .breast hnn Aj nr.. F /nimn UM. The floor 

 or base of the cone is formed by a broad, flat muscle, the 

 diaphragm^ which strejbchesjEicrass the body, and divides the 

 thoracic or chest cavity from the abdominal cavity. 



U. The Ribs. The 

 greater portion of the chest 

 walls is formed by the ribs. 

 They are twenty-four in 

 number, twelve on each 

 side. They do not lie close 

 to one another, the space 

 between being occupied by 

 muscles running obliquely 

 to and fro. These mus- 

 cles assist in expanding the 

 chest. Thejqbs are firmly 

 attached behind to* the spi- 

 nal column. They curve 

 forward, and are joined to 

 the sternum by elastic car- 

 tilage. Those at the 

 are short, with a greater curve ; at the middle of the chest 

 the ribs are long and bowed. The_-4wtr"tower ribs on each 

 side have no attachment in front, and are hence called the 

 floating ribs. The great function of the chest walls, after 

 giving protection to important organs within, is to expand 

 and contract, thus increasing and diminishing the capacity 

 of the chest, and thereby carrying on the life-long process 

 of breathing. The variation of space in the cavity is effected 



FIG. 7. The Ribs and Sternum. 



