=The Respiratory Muscles.=--(Repeat Exps. 13, 14, 15.) The chief breathing muscles are the _diaphragm_ (see Figs. 73 and 74), the muscles _forming the abdominal walls_ (see Fig. 44), and _two sets of short muscles_ (an internal and an external set), _between the ribs_. They are called _intercostals_. (They are the flesh eaten when eating pork ribs.) The _diaphragm_, which is shaped like a bowl turned upside down, rounds up under the base of the lungs somewhat like a dome and separates the chest from the abdomen. Its hollow side is toward the abdomen and its edges are attached to the lowest ribs and the vertebra of the loins. Inspiration is brought about by the rising of the ribs and the descent of the diaphragm. Expiration takes place when the ribs descend, the abdominal walls draw in, and the transmitted pressure lifts the relaxed diaphragm. [Illustration: FIG. 74.--DIAPHRAGM (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).] =Inspiration.=--To cause inspiration the diaphragm contracts, it flattens and descends, since its edges are attached 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 atmosphere has a height of many miles, and the air above is pressing on that below. When the chest walls are raised there would be an empty space or vacuum between these walls and the lungs, did not the pressure of _the outside air push air through the windpipe into the lungs and expand them_ (Exp. 19). [Illustration: FIG. 75.--FRAMEWORK OF CHEST.] =Expiration.=--In very active breathing the abdominal walls actively contract so that they press strongly upon the digestive organs, which in turn _press the diaphragm up. The ribs are also drawn down and in_. Thus the chest becomes smaller and forces the air to flow out through the windpipe (Exps. 20 and 21). [Illustration: FIG. 76.--BLACKBOARD SKETCH, to show how the chest is expanded when the ribs move upward and outward.] THOUGHT QUESTIONS.--_Why breathing with the waist is easier than breathing with the upper chest. Effects of confining the waist_.