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 Experiment 19. How the Ribs are Lifted.—Make a model like sketch to represent backbone, breastbone, and two ribs, using pins to make joints loose at corners. Use cords for diagonals. What happens when cord ac is pulled? When cord bd is pulled? The cords correspond to the two sets of muscles between the ribs.

Experiment 20. Study of Laughing.—Place the hands upon the waist and abdomen when laughing. What motion occurs at each sound of laugh? Draw in the abdominal wall with a jerk. What is the effect upon the breath?

Experiment 21. Modifications of the Breath.—Write I, E, or IE after each word in this list, according as inspiration, expiration, or both, are involved in the action. (Test with sham acts if possible.) Sighing, sobbing, crying (of a child), coughing, laughing, yawning, sneezing, hiccoughing, snoring.

Experiment 22. Effects of Exercise.—Count and record the rates of breathing before and after vigorous exercise.

Experiment 23. Comparative Study.—Observe and record the rate and manner of breathing of cow, horse, dog, cat, etc. Is the air drawn in or sent out more quickly? Is there a pause? If so, after which stage of breathing?

24. Emergency Drill.—Resuscitation from drowning, etc. See Coleman's "Elements of Physiology," page 356.

Necessity for Breathing and for Specialized Organs of Breathing.—The body is a self-regulating machine which possesses energy. This energy, like that of steam engines, arises from oxidation which takes place continually, but at a varying rate. Food for fuel is taken at intervals, but oxygen must be taken in continually. Man breathes about eighteen times per minute. The blood in the tissues soon becomes dark because of loss of oxygen and absorption of carbon dioxid. It is then pumped through the heart to the organ which has the function of absorbing oxygen and giving off carbon dioxid (Fig. 67). In some animals, as the ameba and the earthworm, the surface of the body suffices for breathing. This cell breathing is the true essential respiration; it is universal among living things,