How did heart come to refer to the shape?

Enlarged Heart (Cardiomegaly): What It Is, Symptoms & Treatment

Where Does the Heart Shape Come From? - Origin of the Heart Symbol

Red Heart shape at white background, Clip Art free image download - Clip Art Library

A Brief History of the Heart Shape - Guideposts

Bi-Color Tourmaline Hand Carved Heart Shape Briolette for Jewelry

How the heart became ♥

A Matter of Heart: Where We Got the Heart Shape

How did the human heart become associated with love? And how did it turn into the shape we know today?