Insights from "The Manga Guide to Physiology"

Nice, thanks!

Can’t you infer changes in gravity’s direction from signals from the semicircular canals?

If it helps, back in my military industrial complex days, I wound up excessively familiar with inertial navigation systems. An INS needs six measurements: rotation measurement along three axes (gyroscopes), and acceleration measurement along three axes (accelerometers).

In theory, if you have all six of those sensors with perfect precision and accuracy, and you perfectly initialize the position and velocity and orientation of the sensor, and you also have a perfect map of the gravitational field, then an INS can navigate perfectly forever without ever having to look at its surroundings to “get its bearings”.

Three measurements doesn’t work. You need all six.

I’m not sure whether animals with compound eyes (like dragonflies) have multiple fovea, or if that’s just not a sensible question.

If it helps, back in my optical physics postdoc days, I spent a day or two compiling some fun facts and terrifying animal pictures into a quick tour of animal vision: https://sjbyrnes.com/AnimalVisionJournalClub2015.pdf

As the above image may make obvious, the lens focuses light onto a point. That point lands on the fovea. So I guess you’d need several lenses to concentrate light on several different fovea, which probably isn’t worth the hassle? I’m still confused as to the final details.

No, the lens focuses light into an extended image on the back of the eye. Different parts of the retina capture different part of that extended image. Any one part of what you’re looking at (e.g. the corner of the table) at any particular moment, sends out light that gets focused to one point (unless you have blurry vision), but the fleck of dirt on top of the table sends out light that gets focused to a slightly different point.

In theory, your whole retina could have rods and cones packed as densely as the fovea does. My guess is, there wouldn’t be much benefit to compensate for the cost. The cost is not just extra rods and cones, but more importantly brain real estate to analyze it. A smaller area of dense rods and cones plus saccades that move it around are evidently good enough. (I think gemini’s answer is not great btw.)

Osmotic pressure seems weird

One way to think about it is, there are constantly water molecules bumping into the membrane from the left, and passing through to the right, and there are constantly water molecules bumping into the membrane from the right, and passing through to the left. Water will flow until those rates are equal. If the right side is saltier, then that reduces how often the water molecules on the right bump into the membrane, because that real estate is sometimes occupied by a salt ion. But if the pressure on the right is higher, that can compensate.

>The semicircular canals track changes in your head’s orientation. The otoliths track which way is down. But why not just combine them? Why did they evolve to be separate?

Here’s an idea.

The body is completely obsessed with inferring its state of poisonedness, and uses inner ear orientation sensors to help infer this. This is why car / sea / VR sickness exist. Since inferring poisonedness quickly is important, so it can start forcing itself to throw up, having two sensors is better because.. it’s more.. fault-tolerant? Not sure. But maybe there’s something here.