226 



MOTION AND COORDINATION 



the bones of the thorax form a mechanical device, or 

 machine, for breathing. The ribs are so arranged that 

 the volume of the thorax is increased by elevating them 

 and diminished by depressing them, enabling the air to 

 be forced into and out of the lungs. 



4. The Shoulder and Pelvic Girdles. These groups 

 form two bony supports one at the upper and the other at 



the lower portion of the 

 trunk which serve for 

 |j.S> the attachment of the 

 arms and legs (Fig. 101). 

 The shoulder girdle is 

 formed by four bones 

 two clavicles, or collar 

 bones, and two scapulae, 

 or shoulder blades. The 

 clavicle on either side 

 connects with the upper 

 end of the sternum and 

 serves as a brace for the 

 shoulder, while the scap- 

 ula forms a socket for the 

 humerus (the large bone 

 of the arm) and supplies 

 many places for the at- 

 tachment of muscles. 



The pelvic girdle con- 

 sists of two large bones of 



irregular shape, called the innominate bones. Tliey connect 

 behind with the sacrum and in front they connect, through 

 a small pad of cartilage, with each other. On the inside of 

 the girdle is a smooth, basin-shaped support for the contents 

 of the abdomen, but on the outside the bones are rough 



Pelvic 

 Girdle 



FIG. 101. Bone groups of trunk. 



