The ribs are not horizontal, but slope downwards from the backbone, so that when raised or depressed by the strong intercostal muscles, the size of the chest is alternately increased or diminished.

Thus we have the recti (straight), and the deltoid ([Greek: D], delta), the brachial (arm), pectoral (breast), and the intercostals (between the ribs), so named from their position.

backward movements of head and shoulder, Intercostals. raise and depress the ribs.

The spaces between the ribs are occupied by the intercostal muscles, while large muscles clothe the entire chest.

The walls of the chest being movable, by the contractions of the intercostals and other muscles, the ribs are raised and the breastbone pushed forward.

To illustrate the action of the intercostal muscles (see sec. 210).

The action of the intercostal muscles is not at first easy to understand; but it will be readily comprehended by reference to a model such as that represented in Fig.

The elastic band F G represents the external intercostal muscles, and E H, the internal intercostals.

The elastic band F G represents the external intercostal muscles, and E H, the internal intercostals.