Of all of Leonardo's talents, abilities, and ideas, I believe that his study of anatomy was the most fascinating and most advanced.
He was one of the earliest artists to study anatomy in detail, and his fascination with anatomy added realism to all of his works. In 1476 Andrea Del Verrochio was commisioned to produce a bronze statue of a famous general seated on a horse. Leonardo returned to help, and over the next few months studied equine anatomy in great detail. He filled hundreds of pages with sketches of horses in every pose. Here are a few examples: