*To show Changes in the Thoracic Cavity.*—1. To a yard- or meter-stick, attach two vertical strips, each about eight inches long, as shown in Fig. 51. The piece at the end should be secured firmly in place by screws or nails. The other should be movable. With this contrivance measure the sideward and forward expansion of a boy’s thorax. Take the diameter first during a complete inspiration and then during a complete expiration, reading the difference. Compare the forward with the sideward expansion. [Fig. 51] Fig. 51—*Apparatus* for measuring chest expansion. 2. With a tape-line take the circumference of the chest when all the air possible has been expelled from the lungs. Take it again when the lungs have been fully inflated. The difference is now read as the chest expansion. [Fig. 52] Fig. 52—*Simple apparatus* for illustrating the action of the diaphragm. *To illustrate the Action of the Diaphragm.*—Remove the bottom from a large bottle having a small neck. (Scratch a deep mark with a file and hold on the end of this mark a hot poker. When the glass cracks, lead the crack around the bottle by heating about one half inch in advance of it.) Place the bottle in a large glass jar filled two thirds full of water (Fig. 52). Let the space above the water represent the chest cavity and the water surface represent the diaphragm. Raise the bottle, noting that the water falls, thereby increasing the space and causing air to enter. Then lower the bottle, noting the opposite effect. To show the movement of the air in and out of the bottle, hold with the hand (or arrange a support for) a burning splinter over the mouth of the bottle. *To estimate the Capacity of the Lungs.*—Breathing as naturally as possible, expel the air into a spirometer (lung tester) during a period, say of ten respirations (Fig. 53). Note the total amount of air exhaled and the number of "breaths" and calculate the amount of air exhaled at each breath. This is called the _tidal_ air. [Fig. 53] Fig. 53—*Apparatus* (spirometer) for measuring the capacity of the lungs. 2. After an ordinary inspiration empty the lungs as completely as possible into the spirometer, noting the quantity exhaled. This amount, less the tidal air, is known as the _reserve_ air. The air which is now left in the lungs is called the _residual_ air. On the theory that this is equal in amount to the reserve air, calculate the capacity of the lungs in an ordinary inspiration. 3. Now fill the lungs to the full expansion of the chest and empty them as completely as possible into the spirometer, noting the amount expelled. This, less the tidal air and the reserve air, is called the _complemental_ air. Now calculate the total capacity of the lungs. CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY