Experiment 121. “A substitute for a clinical thermometer may be readily contrived by taking an ordinary house thermometer from its tin case, and cutting off the lower part of the scale so that the bulb may project freely. With this instrument the pupils may take their own and each other’s temperatures, and it will be found that whatever the season of the year or the temperature of the room, the thermometer in the mouth will record about 99° F. Care must, of course, be taken to keep the thermometer in the mouth till it ceases to rise, and to read while it is still in position.”—Professor H. P. Bowditch. Experiment 122. _To illustrate the manner in which the movements of inspiration cause the air to enter the lungs._ Fit up an apparatus, as represented in Fig. 95, in which a stout glass tube is provided with a sound cork, B, and also an air-tight piston, D, resembling that of an ordinary syringe. A short tube, A, passing through the cork, has a small India-rubber bag, C, tied to it. Fit the cork in the tube while the piston is near the top. Now, by lowering the piston we increase the capacity of the cavity containing the bag. The pressure outside the bag is thus lowered, and air rushes into it through the tube, A, till a balance is restored. The bag is thus stretched. As soon as we let go the piston, the elasticity of the bag, being free to act, Movements of drives out the air just taken in, and the piston returns to its former place. Illustration: Fig. 95. Apparatus for Illustrating the Movements of Respiration. It will be noticed that in this experiment the elastic bag and its tube represent the lungs and trachea; and the glass vessel enclosing it, the thorax. For additional experiments on the mechanics of respiration, see Chapter XV. Chapter IX. The Skin and the Kidneys. 232. The Elimination of Waste Products. We have traced the food from the alimentary canal into the blood. We have learned that various food materials, prepared by the digestive processes, are taken up by the branches of the portal vein, or by the lymphatics, and carried into the blood current. The nutritive material thus absorbed is conveyed by the blood plasma and the lymph to the various tissues to provide them with nourishment. We have learned also that oxygen, taken up in the air cells of the lungs, is being continually carried to the tissues, and that the blood is purified by being deprived in the lungs of its excess of carbon dioxid. From this tissue activity, which is mainly oxidation, are formed certain waste products which, as we have seen, are absorbed by the capillaries and lymphatics and carried into the venous circulation.