The principal muscles of inspiration may be assisted by the

I. Serratus posticus superior.

II. Serratus magnus.

III. Pectoralis major.

IV. Pectoralis minor.

The principal muscles of expiration may be aided by the following

muscles:—

I. Serratus posticus inferior,

II. Longissimus dorsi,

III. Sacro lumbalis,

and all the muscles which tend to depress the ribs.

[Illustration: =THE DIAPHRAGM.=]