Its walls are formed partly by the backbone and the ribs and partly by flesh.

A fleshy wall divides the hollow of the trunk into two parts, an upper chamber called the _chest_, and a lower called the _abdomen_.
~8.

The Lungs and Heart.~--The chest contains a pair of organs called the _lungs_, with which we breathe.

It also contains something which we can feel beating at the left side.

This is the _heart_.