From each nostril, three small passages lead backward through the nose.

At the back part of the nasal cavity the passages of the two sides of the nose come together in an open space, just behind the soft curtain which hangs down at the back part of the mouth.

This curtain is called the _soft palate_.

Through the opening behind this curtain the air passes down into the voice-box and then into the lungs.
~18.

The Pleura.~--In the chest the air tubes and lung of each side are enclosed in a very thin covering, called the _pleura_.