[www.loc.gov]

No one knows for sure why a domestic cat purrs, but many people interpret the sound as one of contentment. Our understanding of how a domestic cat purrs is becoming more complete; most scientists agree that the larynx (voice box), laryngeal muscles, and a neural oscillator are involved.

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Continues at [www.loc.gov] with cute pictures. Apparently someone once thought it was a resonance of the heart and blood pumping through it, although I cannot conceive how they came up with that idea…