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.=] DIAPHRAGM. The diaphragm separates the cavity of the thorax from the cavity of the abdomen, and constitutes the floor for the heart and lungs to rest upon, and also a close-fitting cover for the contents of the abdomen. Therefore it is evident that the moving of the diaphragm moves the organs which are immediately above and those below it. In reposeful breathing the enlargement of the cavity of the chest is chiefly accomplished by the contraction of the diaphragm. As it contracts it presses upon the abdominal viscera. The abdominal muscles antagonize the diaphragm by pressing back the abdominal viscera, thus causing its ascent as soon as the diaphragm has become relaxed. As the diaphragm contracts, the air rushes through the nostrils or mouth to fill the lungs. By lifting the ribs the thorax can be sufficiently enlarged to meet ordinary demands for breath; therefore the lungs would not immediately suffer if the diaphragm was not contracted. The principal sufferers in such a case would be the stomach, liver, and intestines, for without this exercise which the contraction of the diaphragm gives them they would not as vigorously perform their functions.