a smooth membrane called the pleura. This membrane turns back and lines the chest wall, so that when the chest expands, the two sleek membranes glide over each other with far less friction than would be the case if the lungs and chest wall were touching (Exp. 2).
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.
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Fig. 73.—Vertical Section of Trunk, showing diaphragm, cavities of thorax and abdomen.
Inspiration.—To cause inspiration the diaphragm contracts, it flattens and descends, since its edges are attached
