Page:First course in biology (IA firstcourseinbio00bailrich).pdf/544

 (or midriff), seen from below. (Cunningham.)

The central portion (light) is tendinous. As the diaphragm descends, it acts like the piston of a great pump and the blood is forced up through the vena cava, and the lymph through the thoracic duct (Fig. 66).

lower than its middle (Fig. 73); the lungs descend with it, thus lengthening the chest from top to bottom; at the same time the ribs are raised upward and outward (Fig. 76) by the contraction of the outer set of muscles between the ribs. Thus the chest is made longer, broader, and deeper from front to back. The lungs expand when the chest expands, and the air rushes in. Why is this? The lungs contain no muscles and cannot expand themselves; the air cannot be pulled in, for its parts do not stick together. The true reason is that the air has weight. The