An inspirational story or nostalgic movie is often said to "warm the cockles of your heart," but where exactly are they located? Not surprisingly, the answer will not be a test question on any medical school exam. The "cockles" are more metaphorical than physical, although the phrase can be traced back to 15th century medical beliefs. Unfortunately, the etymological path gets a tad murky after that.
Under one popular theory, the phrase "cockles of your heart" is derived from the Latin description for the heart's chambers, cochleae cordis. It is believed that the word "cockles" is a corrupted version of cochleae, most likely entering the popular vernacular as a form of slang. The prevailing medical opinion of that day and time was that the ventricles of the human heart resembled the concentric shells of small mollusks or snails, also known as cochleae or cockles. This theory concerning the origin of the phrase does address the connection between the physical and emotional role of the heart, but the shell analogy appears to be more accurate with the structure of the human ear. The Latin cochlea is still used to describe the ear, not the cardium, or heart.
Another theory puts the snail before the cart, as it were. During the Middle Ages, there was an abundance of small mollusks and snails whose shells were vaguely heart-shaped. In the old Irish folk song "Molly Malone," a reference is made to these edible mollusks as "cockles and mussels." It is possible that the shape of these cockle shells inspired a comparison to the chambers of the human heart. This theory sounds plausible, but the Latin root for the mollusks and the Latin root for the heart are not similar. If the phrase "cockles of your heart" did come from a comparison to mollusk shells, then it may have been a form of slang all along.
One possibility of the origin of the phrase may be an alternative definition of "cockles." Some say the chambers of a kiln were called cockles, although that usage has apparently fallen out of common use. Under this theory, the cockles of your heart are analogous to the cold chambers of a kiln, which must be warmed to a certain temperature in order to function at its best. It could be argued that a nostalgic movie or other life-affirming experience warms a person in the same sense that a fire warms the "cockles" of a kiln.
There is even a theory that the French word for shell, coquille, is so close in pronunciation and meaning to "cockle" that a comparison to the shell-like chambers of the human heart was inevitable. Using foreign words in casual conversation is an age-old practice in any language, and it is possible that the phrase "cockles of your heart" may have evolved from the more affected "coquilles of your heart."