(The), in running. All animals soon after the start get out of breath, but as the body becomes heated, breathing becomes more easy, and endures till fatigue produces exhaustion; this is called the second wind.
“That mysterious physical readjustment, known in animals as `second breath,' came to the rescue of his fainting frame.” —The Barton Experiment, chap. x.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894