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I’ve wondered ever since fourth grade (where an anatomical model in a corner of the classroom always made it clear that the heart is centrally located) how the vernacular conception of the heart as located on the left originated and persisted.

Your post finally made it click for me – the aorta extending to the left gives the superficial impression of that being the heart’s location because it’s easier to feel the heartbeat through the skin, versus the more deeply embedded vena cava on the right.

Presumably this means, evolutionarily, greater vulnerability on the left, predisposing the left hand to shielding duties, leaving the right to more dexterous tasks like spearing. The cardiological hypothesis of right-handedness holds!

by cortesoft10 hours ago|

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> Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

I feel like this isn’t really an argument against the theory. If right handedness did evolve because of heart position, a later genetic mutation to have the heart on the opposite side wouldn’t suddenly undo the previous evolution towards right handedness.

by yawpitch6 hours ago|

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Why are you assuming situs inversus, which occurs in species with no handedness (or, indeed, hands) came after handedness?

The argument is that the selection bias was towards precision and the hypothesis was that precision is influenced by heart position (which is, still, in the middle in humans)… individuals with situs inversus would be more precise in the left hand, thus if the causal hypothesis is correct AND the argument holds then there should be a selection bias that would result in a correlation between situs inversus presence and left-handedness.

In the end I don’t believe either the argument or the hypothesis hold even as much water as I can in either hand.

by throwway12038510 hours ago|

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It might be hard to eliminate confounding factors depending on when the research was done. A lot of people in my generation were still dissuaded pretty heavily from writing with their left hands. I'm not entirely convinced anymore as a lay person that "handedness" is a real, distinct phenomenon that's primarily genetically determined or a result of the organization of the brain. It's equally possible that it's a learned preference and that the way the brain organizes around it is as a result of the preference's impact on how you have to solve problems with your preferred hand in a society that preferences right-handedness.

by 11 hours ago|

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by rybosworld11 hours ago|

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Not disagreeing that handedness is probably unrelated to heart position.

But why would situs inversus somehow be tied to this at all? If there's a gene that favors right-handedness, it's not like it would somehow "choose" left-handedness because the individual has their internal organs flipped.

by yawpitch5 hours ago|

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Genes don’t favor (or not favor), but if a natural selection bias for precise dexterity exists AND heart lateral orientation affects dexterity precision THEN those with flipped lateral orientation should exhibit more dexterity in the left hand, thus they should be naturally selected for because of the same bias.

Now, I’d seriously doubt there’s any evidence whatsoever for the assumed selection bias in the first place, never mind any causal relationship between fine motor control and heart asymmetry, but the selection bias should apply to both flips of the anatomical mirror.

by stackghost11 hours ago|

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>Here’s my multiple years of anatomy classes response: the heart isn’t on the left.

Why is the left lung smaller, then?

by rolph11 hours ago|

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not only smaller but having 2 lobes rather than 3, the left lung is possessed of a featureknown as the cardiac "notch" an involution of the lobe that corresponds to the larger left ventricle of the heart.

by altruios11 hours ago|

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More piping to and from the heart exists on the left instead of the right?

by rolph11 hours ago|

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the Aorta and Vena Cava are muchmore central than sinistral.

the aortic arch begins decent left of the coronary corpus, but becomes centralized, tandem with the Vena Cava.

by yawpitch11 hours ago|

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The heart is asymmetrical, but it’s in roughly the center of the chest. The left auricle and ventricle are larger muscles because they’re pumping through the descending aorta to the extremities, that’s the systemic circulatory branch, the plumbing for which is also largely to the right, while the right are pumping into the lungs alone as part of the pulmonary circulatory branch. The left lung (right on those with situs inversus) has two lobes and basically accommodates the extra muscle mass on its side of the heart, but if you really want to kill someone you stab them through the sternum, kind of dead center, not where they hold their hand when performing patriotism.

by stackghost10 hours ago|

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>if you really want to kill someone you stab them through the sternum, kind of dead center, not where they hold their hand when performing patriotism.

Noted, thanks.

by rolph10 hours ago|

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even this is wrong, a penetrating weapon aimed for the heart is applied below the sternum at roughly the positionof the 3rd shirt button, and thrust upward at shallow angle topass behind the manubrium, and is then levered into a pommel upward position so as to lacerate the heart

by yawpitch5 hours ago|

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First, that’s because you want to keep your weapon, which implies you don’t really want to kill the killee. I’m assuming a half inch drill, and I’m leaving it powered up and spinning.

Second, note that what you don’t do when trying to hit the heart is aim left.

by thaumasiotes7 hours ago|

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Well, yes, the point of the solid bone plate right in front of your heart is to block stabbings. And it works!

If you had a weapon that wasn't bothered by the presence of the sternum, and you wanted to stab the heart, you'd go right through the sternum.

by rolph6 hours ago|

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the risk is one of being unable to extract the weapon expediently.

there is an unacceptable risk of having to abandon it.