_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? _Experiment 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, both plants and animals. _To supply the deeper cells large animals require a breathing surface greater than the area of the skin. This is supplied by having the oxygen-absorbing surface folded inward to form folds, tubes, and cavities_ of great complexity. If the lungs of a man were unfolded and all their tubes and cavities spread upon one surface, an area of more than one hundred square feet (or ten feet square) would be covered. Each =respiration=, or breath, consists of the passing in of the air, or _inspiration_, sending it out, or _expiration_, and a _pause_ after one but not after both of the other stages. [Illustration: FIG. 67.--CIRCULATION THROUGH LUNGS (schematic): “venous” blood (in pulmonary artery) black; “arterial” blood (in pulmonary veins) white.] [Illustration: FIG. 68.--OPEN MOUTH, showing palate and tonsils.] [Illustration: FIG. 69.--LUNGS, _P_; with trachea, _TA_; thyroid gland, _th_; larynx, _L_; and hyoid bone, _H_.]