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.

In their passage through the blood and tissues, the albumens, sugars, starches, and fats are converted into carbon dioxid, water, and urea, or some closely allied body.  Certain articles of food also contain small amounts of sulphur and phosphorus, which undergo oxidation into sulphates and phosphates.  We speak, then, of carbon dioxid, salts, and water as waste products of the animal economy.  These leave the body by one of the three main channels,—­the lungs, the skin, or the kidneys.

The elimination of these products is brought about by a special apparatus called organs of excretion.  The worn-out substances themselves are called excretions, as opposed to secretions, which are elaborated for use in the body. (See note, p. 121.) As already shown, the lungs are the main channels for the elimination of carbon dioxid, and of a portion of water as vapor.  By the skin the body gets rid of a small portion of salts, a little carbon dioxid, and a large amount of water in the form of perspiration.  From the kidneys are eliminated nearly all the urea and allied bodies, the main portion of the salts, and a large amount of water.  In fact, practically all the nitrogenous waste leaves the body by the kidneys.