Page:First course in biology (IA firstcourseinbio00bailrich).pdf/545



atmosphere has a height of many miles, and the air above is pressing on that below. When the chest walls are raised there would be an empty space or vacuum between these walls and the lungs, did not the pressure of the outside air push air through the windpipe into the lungs and expand them (Exp. 19).

Expiration.—In very active breathing the abdominal walls actively contract so that they press strongly upon the digestive organs, which in turn ''press the diaphragm up. The ribs are also drawn down and in''. Thus the chest becomes smaller and forces the air to flow out through the windpipe (Exps. 20 and 21).

, to show how the chest is expanded when the ribs move upward and outward.

—''Why breathing with the waist is easier than breathing with the upper chest. Effects of confining the waist''.

1. There are two pairs of ribs below, while there are none