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.
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Fig. 67.—Circulation through Lungs (schematic): "venous" blood (in pulmonary artery) black; "arterial" blood (in pulmonary veins) white.
The Air Passages.—The air usually passes in at the nose and returns by the same way, except during talking or singing. Observe your mouth with a mirror (Fig. 68); at the back part, an arch is seen which is the rear boundary line of the mouth (Exp. 1). Just above the arch is likewise the rear boundary line of the nasal passages. The funnel-shaped cavity beyond, into which both the mouth and nasal passages open, is called the pharynx (far'inks), or throat (see Fig. 68, also Fig. 83). Below,
