Why did William Harvey observe cold-blooded animals and animals near death?

William Harvey observed cold-blooded animals and animals near death specifically because their heartbeats were slow. By studying these animals, he was able to clearly observe the heartbeat and determine that the active phase of the heart—when the muscles contract—is when it decreases its internal volume and expels blood with significant force. This observation was crucial in correcting the previous, incorrect belief that the heart's active phase involved drawing blood into itself by increasing its internal volume.1